There are a bunch of different masts used on forklifts. You would need a deck number in order to possibly find manufacturer's disassembly instructions on the web. It's usually stamped on the rails somewhere.

for many uprights the general removal procedure goes:
disconnect carriage chains on one end
activate lift and raise rails up high enough to get carriage out, or back truck up
chain the rails up and lower the lift cylinder all the way down
remove lift cylinder

Course, there are often things in the way and you have to figure out what must be removed and in what order things get done.

If you have no experience this is probably a job you want to avoid. Rails can chop off any body part that gets in their way. No forklift is worth dying for, not even a Hyster.