Hello, Vault Hunters!
If you already own Borderlands on Steam and are looking to play the upgraded version (AKA Borderlands: Game of the Year Edition Enhanced), it's a simple process to get yourself set up.
Provided Borderlands or the original Borderlands: Game of the Year Edition is already in your Steam Library, the new version of the game should download already. Just open up Steam and check out your Library. You should see Borderlands GOTY Enhanced sitting in the list, right below Borderlands GOTY.
Please understand: This is a brand new title that you'll have to install onto your computer, not an upgrade for your existing copy of Borderlands.
Go ahead and select Borderlands GOTY Enhanced and install it. Once you've done so, you can play the game.