PM> Install-Package Spire.Presentation

using Spire.Presentation;

namespace AddSpeakerNotes
{
    internal class Program
    {
        static void Main(string[] args)
        {
            //Load a PowerPoint document
            Presentation ppt = new Presentation();
            ppt.LoadFromFile("Sample.pptx");

            //Get the first slide
            ISlide slide = ppt.Slides[0];
            //Add a notes slide
            NotesSlide notesSlide = slide.AddNotesSlide();

            //Add a paragraph to the notes slide
            TextParagraph paragraph = new TextParagraph();
            paragraph.Text = "Tips for making effective presentations:";
            notesSlide.NotesTextFrame.Paragraphs.Append(paragraph);

            //Add a paragraph to the notes slide
            paragraph = new TextParagraph();
            paragraph.Text = "Use the slide master feature to create a consistent and simple design template.";
            notesSlide.NotesTextFrame.Paragraphs.Append(paragraph);

            //Add a paragraph to the notes slide
            paragraph = new TextParagraph(); 
            paragraph.Text = "Simplify and limit the number of words on each screen.";
            notesSlide.NotesTextFrame.Paragraphs.Append(paragraph);

            //Add a paragraph to the notes slide
            paragraph = new TextParagraph();
            paragraph.Text = "Use contrasting colors for text and background.";
            notesSlide.NotesTextFrame.Paragraphs.Append(paragraph);

            //Set the bullet type and bullet style for specific paragraphs on the notes slide 
            for (int i = 2; i < notesSlide.NotesTextFrame.Paragraphs.Count;i++)
            {
                notesSlide.NotesTextFrame.Paragraphs[i].BulletType = TextBulletType.Numbered;
                notesSlide.NotesTextFrame.Paragraphs[i].BulletStyle = NumberedBulletStyle.BulletArabicPeriod;
            }

            //Save the result document
            ppt.SaveToFile("SpeakerNotes.pptx", FileFormat.Pptx2013);
        }
    }
}

Imports Spire.Presentation

Namespace AddSpeakerNotes
    Friend Class Program
        Private Shared Sub Main(ByVal args As String())
            'Load a PowerPoint document
            Dim ppt As Presentation = New Presentation()
            ppt.LoadFromFile("Sample.pptx")

            'Get the first slide
            Dim slide As ISlide = ppt.Slides(0)
            'Add a notes slide
            Dim notesSlide As NotesSlide = slide.AddNotesSlide()

            'Add a paragraph to the notes slide
            Dim paragraph As TextParagraph = New TextParagraph()
            paragraph.Text = "Tips for making effective presentations:"
            notesSlide.NotesTextFrame.Paragraphs.Append(paragraph)

            'Add a paragraph to the notes slide
            paragraph = New TextParagraph()
            paragraph.Text = "Use the slide master feature to create a consistent and simple design template."
            notesSlide.NotesTextFrame.Paragraphs.Append(paragraph)

            'Add a paragraph to the notes slide
            paragraph = New TextParagraph()
            paragraph.Text = "Simplify and limit the number of words on each screen."
            notesSlide.NotesTextFrame.Paragraphs.Append(paragraph)

            'Add a paragraph to the notes slide
            paragraph = New TextParagraph()
            paragraph.Text = "Use contrasting colors for text and background."
            notesSlide.NotesTextFrame.Paragraphs.Append(paragraph)

            'Set the bullet type and bullet style for specific paragraphs on the notes slide 
            For i As Integer = 2 To notesSlide.NotesTextFrame.Paragraphs.Count - 1
                notesSlide.NotesTextFrame.Paragraphs(i).BulletType = TextBulletType.Numbered
                notesSlide.NotesTextFrame.Paragraphs(i).BulletStyle = NumberedBulletStyle.BulletArabicPeriod
            Next

            'Save the result document
            ppt.SaveToFile("SpeakerNotes.pptx", FileFormat.Pptx2013)
        End Sub
    End Class
End Namespace

using Spire.Presentation;
using System.Text;
using System.IO;

namespace ReadSpeakerNotes
{
    internal class Program
    {
        static void Main(string[] args)
        {
            //Load the PowerPoint document
            Presentation ppt = new Presentation();
            ppt.LoadFromFile("SpeakerNotes.pptx");

            //Get the first slide
            ISlide slide = ppt.Slides[0];           

            //Get the notes slide from the first slide
            NotesSlide notesSlide = slide.NotesSlide;
            //Get the speaker notes from the notes slide
            string notes = notesSlide.NotesTextFrame.Text;
            //Create a StringBuilder instance
            StringBuilder sb = new StringBuilder();
            //Append the speaker notes to the string builder
            sb.AppendLine(notes);
            
            //Save to .txt file
            File.WriteAllText("SpeakerNotes.txt", sb.ToString());
        }
    }
}

Imports Spire.Presentation
Imports System.Text
Imports System.IO

Namespace ReadSpeakerNotes
    Friend Class Program
        Private Shared Sub Main(ByVal args As String())
            'Load the PowerPoint document
            Dim ppt As Presentation = New Presentation()
            ppt.LoadFromFile("SpeakerNotes.pptx")

            'Get the first slide
            Dim slide As ISlide = ppt.Slides(0)

            'Get the notes slide from the first slide
            Dim notesSlide As NotesSlide = slide.NotesSlide
            'Get the speaker notes from the notes slide
            Dim notes As String = notesSlide.NotesTextFrame.Text
            'Create a StringBuilder instance
            Dim sb As StringBuilder = New StringBuilder()
            'Append the speaker notes to the string builder
            sb.AppendLine(notes)

            'Save to .txt file
            Call File.WriteAllText("SpeakerNotes.txt", sb.ToString())
        End Sub
    End Class
End Namespace

using Spire.Presentation;
using System.Text;
using System.IO;

namespace DeleteSpeakerNotes
{
    internal class Program
    {
        static void Main(string[] args)
        {
            //Load the PowerPoint document
            Presentation ppt = new Presentation();
            ppt.LoadFromFile("SpeakerNotes.pptx");

            //Get the first slide
            ISlide slide = ppt.Slides[0];
            //Get the notes slide from the slide
            NotesSlide notesSlide = slide.NotesSlide;

            //Remove a specific speaker note from notes slide
            //notesSlide.NotesTextFrame.Paragraphs.RemoveAt(1);

            //Remove all the speaker notes from notes slide
            notesSlide.NotesTextFrame.Paragraphs.Clear();

            //Save the result document
            ppt.SaveToFile("DeleteSpeakerNotes.pptx", FileFormat.Pptx2013);

        }
    }
}

Imports Spire.Presentation

Namespace DeleteSpeakerNotes
    Friend Class Program
        Private Shared Sub Main(ByVal args As String())
            'Load the PowerPoint document
            Dim ppt As Presentation = New Presentation()
            ppt.LoadFromFile("SpeakerNotes.pptx")

            'Get the first slide
            Dim slide As ISlide = ppt.Slides(0)
            'Get the notes slide from the slide
            Dim notesSlide As NotesSlide = slide.NotesSlide

            'Remove a specific speaker note from notes slide
            'notesSlide.NotesTextFrame.Paragraphs.RemoveAt(1);

            'Remove all the speaker notes from notes slide
            notesSlide.NotesTextFrame.Paragraphs.Clear()

            'Save the result document
            ppt.SaveToFile("DeleteSpeakerNotes.pptx", FileFormat.Pptx2013)

        End Sub
    End Class
End Namespace