Sometimes the outer caps stick in the distributor housing. If that happens take the coil off the top and gently tap the caps out using a wooden dowel, a screwdriver handle, or similar. Frequently the easiest disassembly is to get the caps off the distributor, pull at least one side spark plug conduit, then pull the distributor from the engine.
Terry described how to separate the outer cap from the inner cap.
When putting things back together if there is any combination of new inner cap and/or rotor then check for the rotor hitting the caps. About 1/3 of the time I have to file one or more of the brass cap studs for clearance since something hits.