Pull the belt.
Remove the torx head bolt.
Remove the pulley.
Put new pulley on.
Install torx head bolt.
The highest risk in this is stripping the torx head bolt. If that happens, cut the pulley off any way you can (I used a hand grinder with a 4" wheel) and remove the bolt with a pair of vice grips. Remove the idler bracket assembly from the engine, and you will see the tapped hole goes right through to the back. Drill the hole out to clear a 3/8" bolt. Install the new pulley using the bolt with a washer, pass the bolt through the bracket and double nut it from the back side. The washer needs to be large enough in diameter to cover the central hub (that middle part that does not turn) but does not contact the rotating section.
I can walk you through this,
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)