serpentine belt tensioner

Drain the cooling system.
Remove the alternator.
Remove the heater hoses from the drive belt tensioner fittings.
Remove the drive belt tensioner bolts (3).
Remove the drive belt tensioner from the engine.

Installation is the reverse.

See the guide below, not make/model specific but will give you an idea of what it takes

https://www.2carpros.com/articles/serpentine-belt-tensioner-replacement

Hers a link to the page with the belt routing diagram

https://www.2carpros.com/diagrams/buick/lesabre/2003