Not sure if this is the “right” way, but it worked for me. Tip the mower onto its side, carburettor upward. Remove the plastic cover over the drive pulley, it appears to be part of the plastic cutting deck, but look closer, it is separate. Two screws were particularly tricky with corroded. Remove the spring holding the clutch in place. To replace the belt I took off the cover for the smaller driver and loosened the rear roller, I could then squeeze the old belt off and the new belt on. Reverse to put all back together. Worked fine. I also took the opportunity to sharpen the blades.
Stephen
May 2022