Can anyone help?

I have a form which i use to enter information into the
header of a document, when the ok button is clicked i
have to following code:

Private Sub OK_Click()
With ActiveDocument
   .Bookmarks("est").Range _
   .InsertBefore TextBox1
   
End With

UserForm1.Hide

End Sub

the only trouble is if i need to modify a field, i.e i
reopen the form and re enter the data it will only add to
what is already there, for example if i enter 'xxx' in
the first instance and need to change it to 'yyy' it will
be displayed in my header as xxxyyy i need some sort of
code to erase what is already there and re enter the text
when the form is reused.

please forgive me if the answer to this question is a
bit, well, obvious but i am very new to this whole thing!

your help would be massively appreciated!

Regards

Alex