The address frame is where the address is printed. You can drag it anywhere you want on the envelope and in it is where the address will print.
If it is the image of the stamp location you want to move then simply use the envelope tool to create a new document and add a text box where you want the stamp to appear and save it as a template. Create a new document from that template when you need an envelope.
To that end you may find
Envelope or label add-in for Word useful.
You can use the supplied envelope template as a start point - or see attached which will work with that add-in or as a stand-alone template: