Sub UpdateBookMark()
Dim BmkNm As String, NewTxt As String, BmkRng As Range
BmkNm = InputBox("Bookmark Name")
NewTxt = InputBox("New Bookmark Text")
With ActiveDocument
  If .Bookmarks.Exists(BmkNm) Then
    Set BmkRng = .Bookmarks(BmkNm).Range
    BmkRng.Text = NewTxt
    .Bookmarks.Add BmkNm, BmkRng
  Else
    MsgBox "Bookmark: " & BmkNm & " not found."
  End If
End With
Set BmkRng = Nothing
End Sub