FlashBack has separate installations for each edition (Express, Plus and Pro) to allow users to install different editions side-by-side on the same PC.
Since FlashBack licence keys are specific for a particular edition, this means that you will then to install the edition that you have upgraded to. For example, if you upgrade from FlashBack Plus 5 to FlashBack Pro 5, you will need to install FlashBack Pro 5.
If you log into your account at https://secure.blueberryonlinestore.com/Login.aspx, you can download any edition of FlashBack that you have a valid licence for from the Download Product page - this will automatically update to display the correct links specifically for you.
Once you have installed the version that you have upgraded to , you will be prompted to enter your licence key when you first run FlashBack.
(Note - when upgrading a version 1 - 4 licence, you will be issued a new licence key but for version 5 and onwards, the existing licence key will continue to be used)