there's going to be a tensioner arm with a nut in the center. I think there is a diagram on the underside of the hood or on the radiator cover that might show you which one it is. But, just use a socket set to either push it down or pull it up (cant remember which direction) and the belt will get loose enough to slide it off. Install is basically the same way; just start to route the lower pulleys then when you get to the last one or two put pressure back on the tens arm and finish routing the belt.
BTW...I just removed the belt from my folks sable and sprayed and scrubbed the carb cleaner on the idler and tensioner pulleys and the backside of the belt and after putting it back in its whisper quiet now. You can get the carb cleaner at autozone, advance auto, etc. I got the Gumout brand, but any carb cleaner should work.