If it is like the 2000 LeSabre, the rear seat will lift out and the battery is under it on the passenger side. It requires some force, but the front of the seat cushion should lift up then can be removed.
The 2001 Buick LeSabre rear shocks are held in place with a retaining bolt on each end of the shock. Remove the retaining bolts and the old shocks will come off. Reverse the process to install your new rear shocks.