My mast is down now. I could take a picture if you like. If you have no other things attached to your masthead (VHF, instruments, etc), then there is a cap held on with one screw on the front of the mast. When you remove the cap, the axel will lift out and the sheaves can be slid off, new ones slid on, and it all dropped back in place. You have to relieve tension on your halyards, etc. That leads to the issue of how to support yourself once your up. Be safe!
With instruments on you masthead, then you may have to remove other things first to get it appart. My windex and anchor light and raymarine instruments are attached to the removable plate on my mast. In general it is simple to get apart.