It's doable but complicated to do it on Steam. I also play Team Fortress 2 (that cartoon shooter, awesome laughs all around) and the way TF2 betas through Steam work is that you need to install a completely separate version of the game.
It's simpler and takes up less disk space to do it like we currently do. Unrar into game folder, try the patch, if you don't like it tell Steam to "verify integrity of local cache" and it will revert to the previous, non-beta version
