An idler pulley is a pulley that does not drive and is not connected to any device that is driven. It serves to either tension a belt or to route a belt to clear an obstacle.
Get a standard 3/8" socket wrench, just the socket wrench, you don't need any sockets. Look on the idler pulley bracket for a square hole in the bracket. Place the socket driver in the square hole....
This is an extremely eays job and takes just a couple minutes. you need a n apropriate size socket a short extension and a rachet. loosen the bolt through the pulley and remove it and the pulley...
First with a 1/2 drive rachet release the tension and the neopream belt. Then with a socket take out only nut holding the pulley. Replacment will guide and only fit one way.