Undo the screws (14) and remove the sprocket cover (13).
Slacken the chain (Adjusting the chain tension).
Engage a low gear and unscrew the two screws (10) on the sprocket retaining plate (11).
Remove the plate (11) from the gearbox output shaft.
Slide the front sprocket (12) with the chain (9) off the gearbox output shaft and then unhook the chain from the sprocket.