You will have to remove the belt and to do so you use a socket and attach to the bolt on the tensioner and turn it to release the tension on the belt, mine is a 15mm socket. Then slide the belt off the tensioner. Then the idler pulley has a 6mm allen in it and yes, you just loosen it and slide it off when you get the bolt out. It you don't have a 6mm allen socket, then use an allen wrench, and I use my jack handle with an open end to increase the length of the wrench to give more torque to remove the bold. Good luck!!
Regards,
Brian