Is it easy to replace a distributor cap and the rotor on a 1992 Geo Prizm or should a professional do it?

Yes very easy, Just don't get the plug wires mixed up. Take it one wire at a time. Start with #1 which is listed on the cap. The rotor just pulls off and plugs back on. Twist until it goes into the slot.

as long as you are careful and remember exactly how each wire and rotor position was before you take it apart - it is a no brainer. The rotor has a small wedge/cutout that ahs to sit correctly on the shaft. The wires have to be pushed well into the ditributor - that's it.