How do you change the pully on top of the transmission. Or does it have a sheer pin? Can not get my riding lawnmower to go into gear going forward or backwards.

Hi,
It would be wise to have someone else sit on the machine and try peering into the frame ti see if the pulley spins with engine running and clutch disengaged. The pulley is keyed to the shaft. It generally requires the removal of the transaxle from the frame to replace.
I can furnish parts breakdowns and part numbers, and possibly more targeted instruction if given the model number.
Peace,
Clarke