UPDATE:  I figured it out!

'macro to update bookmarks
  Sub UpdateBookmark(BookmarkToUpdate  As String, TextToUse  As String)
    Dim BMRange As Range
    Set BMRange = ActiveDocument.Bookmarks(BookmarkToUpdate).Range
    BMRange.Text = TextToUse
    ActiveDocument.Bookmarks.Add BookmarkToUpdate, BMRange
End Sub
'called like this:
UpdateBookmark "NameOfBookMark", "String to insert"

cheers then,

hillsider

The normal procedure would be to create a new document from the template
each time.  To do what you want to do however, it would be better to used
document variables and DOCVARIABLE fields instead of bookmarks.

In the code behind the user form, you would use

With ActiveDocument
   .Variables("varname1").Value = [somecontrolvalue]
   .Variables("varname2").Value = [someothercontrolvalue]
   etc
   .Range.Fields.Update
End With

and in the template, where you want [somecontrolvalue] to appear, you would
have a { DOCVARIABLE varname1 } field, etc.