You can remove the belt that drives the gearbox at this point - if this has worn out then only the high speed attachment will have been working. It will lift off nicely with a screwdriver.
The large gear then needs to be gripped, and the "dog" on top of it needs to be removed. You'll need to hold the gear in place and turn the dog, and watch the spring doesn't fly off!
Apologies for the cake/belt continuity errors in this set - some are taken during reassembly