How do you loosen the alternator belt when trying to remove the alternator on a 1996 Pontiac Sunfire?

Somewhere on the belt there should be a tensioer that is mouted to the engine or the alternator bracket. Turn it counter clockwise with a 15mm wrench and it should lift the tensioner enough to slide off the belt.

On my '96The tensioner is down and to the left of the alternator (looking at the engine from the passenger side fender)

I had to replace the alternator just this weekend. The tensioner is loosened by putting a 15mm wrench on the center nut and rotating it to the right (toward the front of the car)