I'm not on FTTx but the normal approach:
-Go to the broadband ordering page using the "Order Now" button on http://www.aa.net.uk/broadband.html
-log in with your account number and password
-click on bradband to expand if ncecessary
-click on the address line for your site which will bring up an "edit site options" page
-clicik change your tarrif
-confirm order on order page that appears.
I assume it's similar for FTTx - NB this isn't the same as "upgrading" your package.