It'* easy, and should take less than 15 minutes. The bolts you'll need to loosen/tighten are metric. I think it'* a 15mm socket for the bolt on the belt tensioner, but I don't remember for sure. Rotate that bolt clockwise and keep pressure applied (it'* spring-loaded) and remove the belt while the tension is removed, then let go of the socket wrench slowly to allow the spring to return the bolt to its at-rest position. The 2 bolts on the alternator are easy, and I use a crescent wrench or second socket to hold the nut while I loosen/tighten the rear bolt.
After the alternator has been replaced and the bolts have been reinstalled, release the tension on the tensioner pulley again and put the belt back on.
'99 Bonneville SLE, white, 112,000 miles
'02 Park Avenue Ultra, white diamond pearl, 88,000 miles