void SetText(
    string text
)

Sub SetText(
    text As String
)

using DevExpress.XtraRichEdit;
using DevExpress.XtraRichEdit.API.Native;
using System.Linq;
// ...

document.LoadDocument(@"Documents\Watermarks.docx");

// Add a text watermark to the document.
document.WatermarkManager.SetText("DO NOT COPY");

// Access the first document section.
Section firstSection = document.Sections[0];

// Obtain the section's primary header.
SubDocument headerContent = firstSection.BeginUpdateHeader();
// Retrieve the header's watermark.
var watermark = headerContent.Shapes.SingleOrDefault(x => x.Type == ShapeType.Watermark);
// Change watermark text.
if (watermark != null && watermark.WatermarkFormat.Type == WatermarkType.Text)
{
    watermark.WatermarkFormat.SetText("CONFIDENTIAL");
}
firstSection.EndUpdateHeader(headerContent);
document.SaveDocument(@"Documents\WatermarksUpd.docx", DocumentFormat.Docx);

Imports DevExpress.XtraRichEdit
Imports DevExpress.XtraRichEdit.API.Native
Imports System.Linq
' ...

document.LoadDocument("Documents\Watermarks.docx")

' Add a text watermark to the document.
document.WatermarkManager.SetText("DO NOT COPY")

' Access the first document section.
Dim firstSection As Section = document.Sections(0)

' Obtain the section's primary header.
Dim headerContent As SubDocument = firstSection.BeginUpdateHeader()
' Retrieve the header's watermark.
Dim watermark As Shape = headerContent.Shapes.SingleOrDefault(Function(x) x.Type = ShapeType.Watermark)
' Change watermark text.
If watermark IsNot Nothing AndAlso watermark.WatermarkFormat.Type = WatermarkType.Text Then
    watermark.WatermarkFormat.SetText("CONFIDENTIAL")
End If
firstSection.EndUpdateHeader(headerContent)
document.SaveDocument("Documents\WatermarksUpd.docx", DocumentFormat.Docx)