Going by memory here....
When you are in the label page...where you choose mailing service, date, etc...
Seller address should be up at the top left, there should be the word "edit" that is a link so you can edit the address.
I *think* it pulls the address from the mailing address in your account settings, so you may be able to change it there (if you are looking for it to "stick") and see if it carries over to labels.