It has been a real long time since I have worked on one, and your model is likely newer than the ones I worked on, but most of those all Rear Engine Riders had the blade belt tension adjusted by loosening the center tube clamp and pulling the front of the mower forward while the rear wheels were locked, didn't take much, then re-tighten.
But if you put up your actual mower's model numbers, we should be able to call up a manual specific to your unit.