First, remove the belt.
Then, remove the puller mounting bolt. They pulley will slide off. Note that if you turn the bolt in the wrong direction, you will crack the tensioner and it must be replaced.
If you are using a non-GM replacement part, you may have a 'universal' pulley that doesn't actually fit and now need to add or remove spacers or washers.
Position the new pulley in place of the old one. I recommend applying blue thread locker to the threads of the bolt. Bolt it in. Reinstall the belt.