Can't get information to refresh/replace in a TextBox

Hi All,

I have the following code:

If Me.ComboBox7.Value = "NO HUB" Then

Me.TextBox5 = Left$(Me.TextBox5.Text, 8) & "Z"

Else
Me.TextBox5 = Left$(Me.TextBox5.Text, 8) & Left$(Me.ComboBox7.Text, 1) & Mid$(Me.TextBox5.Text, 9)


End If

This is one of 8 codes which, when selection is made will populate TextBox5 with a specific code. This happens dynamically as opposed to an older method whereby I pressed a button.
The problem I am experiencing is the information will not replace the previous option, but rather sit along side it. Is there an adaption that can be made to this code so If the user changed their mind, it would override the selection?