Amplifiers contain voltages high enough to Kill you even after the amp has been powered Off !!! If you don't have enough electronic experience to identify and discharge the Filter Capacitors which contain these Deadly voltages, take your amp to a Qualified tech. If you don't know a qualified tech in your area, post a request in the alt.guitar.amps newsgroup, the folks there will be happy to help you.
1. Remove and save the original speaker. (Princetons came with terribly inefficient 10" Oxfords).
2. Add a high efficiency 10" speaker like a Reissue Jensen. Make sure the impedance/ohm value matches that of the speaker you removed. And if you want a little more bottom end you can even cut yourself a new baffle out of 3/4" plywood which will accomodate a 12" speaker. Then add new grill cloth. (Don't remove the cloth from the original baffle).
3. Add two 470 ohm resistors to the screen grids of the 6V6 power tubes. (This was a customary practice in all of the higher priced big Fender amps).
3. Using the available schemtics, go through the entire power section and replace all of the components using the highest quality parts you can find. Make sure they match the original values "Exactly".
3. Finally, a slight inherent "mismatch" exists in the Phase Inverter section of the Princeton design. Matching this up will complete the mod.