If you have the Timefall Porter Beer being served in the private room and you wish to switch back to the energy drink, you just need to get Sam intoxicated to the point he clears everything off the table (usually takes 2 trips to the private room making sure you drink all the beer the first trip). After he's cleared the table, leave the room and on your next visit the energy drink will be served. To change back to beer, simply make another delivery of Timefall Porter Beer from the Timefall Farm to that destination.