In Steam, add the game to your library by clicking the Games menu (top bar) and choosing Activate a Product on Steam
Paste in your Steam key to add the game
Play the game!
If you have the ALPHA installed:
- In your library, right click on the game and choose Properties
- Go to the BETAS tab
- If your drop-down says Alpha, switch it to NONE - Opt Out
- The Beta should download and replace the old alpha build!