How do you increase the power on a precharged pneumatic air rifle?

It depends on the model. Some have a dial to adjust the power like the Airforce Condor or the Shinsuin, other like the Marauder do not have simple adjustment dial but can be adjusted with a tool. Each one is different so you have to read the Owners manual to see if it is possible. One simple method is to put some Pellgun oil on your pellets. This will increase the FPS. This even works on Non PCP air gun and rifles.