when you purchase the upgrade, you are given a link to the download page. you then download the files on that page (there are several, depending on which pack you purchased), then just install each of the files you downloaded by doubleclicking them.

RB and BIAB are both updated with those files.

the upgrade will take a while depending on the pack you purchased. this is one of my pet peeves about BIAB installation - I don't understand why each separate file has to be installed separately, but that's the way they chose to implement it.