The bigger Ariens are easy to change the belts on. I would assume the smaller ones are easy too. On the bigger 70s and 80s machines you just remove the belt cover, remove the chute control rod, remove the 2 bolts holding the front and rear together and pivot the 2 pieces on the bottom hinge. It is easier if you have someone to hold the handles, but if you have something to lean it on that works too. No pulleys to remove, the belt should just slip off then. Probably have to hold the auger brake down to release the belt. Oh yea, get the belt off of the engine side before taking the 2 pivot bolts out.
You can get the manuals from Ariens website. They have an owners manual and a maintenance manual if you can get your model number and serial number. They will tell you how to change the belt on your specific blower.
The manuals will also tell you the correct part number for your belt. Once you have the part number some simple google searching should give you the generic size of the belt. You can search online for a good price on an OEM belt, or get a generic. If you choose a cheaper generic one be sure you get a heavy duty / high torque / clutching application / kevlar cord belt. You have a length and a width so make sure both numbers match.