Windows Form: Add header to existing Doc file using VB.Net

Refer below code.

Add as Bookmark.

Dim app As Application = New Microsoft.Office.Interop.Word.Application()
Dim doc As Document = app.Documents.Open("E:\\Test.doc")
Dim bookmark As String = "BookmarkName"
Dim bm As Bookmark = doc.Bookmarks(bookmark)
Dim range As Range = bm.Range
range.Text = "Welcome to ASPSnippets"
doc.Bookmarks.Add(bookmark, range)

Or use Range method.

doc.Range(0, 0).Text = "Welcome to ASPSnippets"