Clear a warm day for the job .. take your time ... have a helper ... be careful if it's one you have to wind the spring.
If you're not accustomed to physical work and a lot of arms-over-head type work .. lay up a cache of Aleve and program your wife/girlfriend to feel sorry for you that she will soothe your aching muscles.
Piece of cake.
No kidding! Have a helper. I just installed a newer garage door mechanism to replace the one that died. Oh, My Arms!!! And that was just the electronic opener. The tricky part, having done the manual garage door replacement thing, a ways back? Was the balancing of the door to parts.