If your firm's existing address needs updating:
Go to the system address book (click "HOME..Address book") and locate your firm's entry (should be highlighted in yellow). Amend the address record for your firm and save it. The new details will now be used.
If your firm's name has changed completely:
Create a new address book record (see "HOME..Address book". Fill out all relevant contact information and then make sure you select the option "Use current address book entry as our firm" under the "Options" page for the address book record.