Reset Visual Studio 2013 Serial - HELP HELP HELP

My friend, uh bob, installed Visual Studio 2013 with a "VERY-LEGITIMATE" serial he got off the internet. He later decided that he should probably get a "EVEN-MORE-VERY-LEGITIMATE" serial for cheap off some nice guy called Sean off some red-it forum thing but he is unsure how to TOTALLY OBLITERATE THE ORIGINAL SERIAL SO AS TO REGISTER VISUAL STUDIO.

Bob is SOOOO DUMB. HELP!

Might be able to uninstall and then install again with the new key. Might depend on whether "bob" has logged in with a Microsoft account or not. I can't remember off the top of my head if the product keys are linked to the accounts in any way. I get mine through DreamSpark for free so I don't worry much about the licencing.