On the shorter end of the drive unit there is a case held in place by 4 bolts. Removed that case and you should see a nut with a cotter pin. Remove that nut and bearing cone. You can now hit that shaft toward the other side. **Do not hit it out all the way**. Once the drive tire is able to fall down, you do not need to take the shaft any father. This shaft goes through a gear in the gearbox and if you fully remove the shaft it can be a pain to get it realigned.
Reverse the steps to install the new drive tire. To line it up and get the shaft all the way back in you will need to twist the shaft until the woodruff key lines up to the slot in the gear.