From our homepage, look at the Downloads menu and pick Upgrades instead of Service Packs. From here, you can download the 'Upgrade' to the same version of Adagio Invoices, which is required to change the version from 81A to 81B. You can go directly to the May 2009 release and bypass any previous releases at the 81B level. Here is a link to the Upgrades web page.

The serial number and install code for this upgrade would have been emailed to you when version 81B was first released, back in March 2009.