I own a 1999 BUICK LESABRE
6 Cylinders 231 K 3.8L SFI and I will like to know if there is anything special I must do to replace the serpertine belt. I have the new belt and I remove the old one by releasing the belt tencioner, but the new oone since to be a little small. The store where I bought it and the mnufature of the belt say i have the right belt.

the new belt will be a little smaller than the old one since the belt on your engine streches over time...
if you stretch the 2 of them out side by side, the new one should be shorter by no more than an inch.

You are going to need at least a 12 inch long box or socket to overcome that pressure on the tentioner spring.

YOu can do it with a 12 inch if you can press 200 pounds or a 16 inch length wrench if you are like me.

Hint it is safer if you have a STRONG friend and do not loosen the tensioner more than necessary to remove the old belt or replace the new one. I don't know if the tensioner has a lockout to hold it fully loose. If it does, you might loose a finger trying to release it if you don't know what you are doing.