public TextArea TextArea { get; }

Public ReadOnly Property TextArea As TextArea

using DevExpress.Docs;
using DevExpress.Docs.Presentation;
using System.Drawing;
//...

using (var presentation = new Presentation(File.ReadAllBytes(@"C:\Documents\Presentation.pptx"))) {
    Slide slide = presentation.Slides[0]; 

    var note = slide.Notes;
    note.TextArea.Text = "updated text";

    // Create paragraph properties
    ParagraphProperties textParagraphProperties = new ParagraphProperties() {
        TextProperties = new TextProperties() { HighlightColor = new OfficeColor(Color.Red) }
    };

    // Apply text properties to the note
    note.TextArea.ParagraphProperties = textParagraphProperties;
    note.TextArea.Level1ParagraphProperties = textParagraphProperties;

    presentation.SaveDocument(new FileStream("C://Documents//presentation_updated.pptx", FileMode.OpenOrCreate, FileAccess.ReadWrite));
}

Imports DevExpress.Docs
Imports DevExpress.Docs.Presentation
Imports System.Drawing

Using presentation = New Presentation(File.ReadAllBytes("C:\Documents\Presentation.pptx"))
    Dim slide As Slide = presentation.Slides(0)

    Dim note = slide.Notes
    note.TextArea.Text = "updated text"

    ' Create paragraph properties
    Dim textParagraphProperties As ParagraphProperties = New ParagraphProperties() With {
        .TextProperties = New TextProperties() With {
                .HighlightColor = New OfficeColor(Color.Red)
            }
    }

    ' Apply text properties to the note
    note.TextArea.ParagraphProperties = textParagraphProperties
    note.TextArea.Level1ParagraphProperties = textParagraphProperties

    ' Save the updated presentation
    Using fs As New FileStream("C:\Documents\presentation_updated.pptx", FileMode.OpenOrCreate, FileAccess.ReadWrite)
        presentation.SaveDocument(fs)
    End Using
End Using