How to Update other Document Text field

[eric leung]

I have one VBA application run on Word, the program generate new document
using template.
In the template have some Text field in the header(Control Toolbox, Text
field), I want to know how to update those field ?

Public Sub Build_doc()
Dim x As Document
Set x = Documents.Add(Template:="Invoice.dot", Visible:=True)
'x.txtInvoice.Value = "ddd"

End Sub