It's a pain. Best to lift up the rear end of the mower to do this , despite what the manual might say. 1. First, ensure that the plug lead is removed and secured so that the engine cannot start by accident. 2. Underneath the front (drive) pulley, there is a plate which protects it. Remove the 3 small bolts and then the plate. 3. There is a piece which supports the end of the drive shaft/pulley, held by another 2 bolts. Undo them. Make sure that the little sleeve which lives in the support piece does not get lost in all the crud that has fallen off. 4. Remove the 3 large bolts securing the engine to the body. 5. Now pull the engine away from the support piece, and tilt the pulley end down. The belt will now have enough space to slide out over the pulley. It can be done by one person but 2 is better (the mower is heavy and unstable).