Sub UpdateBookmarks()
Dim i As Long
With ActiveDocument
  For i = 1 To .Bookmarks.Count
    With .Bookmarks(i)
      Call UpdateBM(.Name, InputBox("Please enter " & .Name))
    End With
  Next
End With
End Sub

Sub UpdateBM(bmName As String, bmContent As String)
Dim bmRng As Word.Range
With ActiveDocument
  Set bmRng = .Bookmarks(bmName).Range
  bmRng.Text = bmContent
  .Bookmarks.Add bmName, bmRng
End With
End Sub