Google. There are handful of sleep apnea forums with instructions on how to enter into the "clinician mode" of most machines. From there, you can adjust modes, pressure levels, etc. Said forums also have wealth of information on WHAT to change. There is also software called "sleepyhead" that is free and downloadable if you really want to geek out and understand the data saved by the machine.

I "hack" (not really, just doing stuff / adjustments that clinics supposed to do) my machine for no other reason than they clinicians are useless once they sell you a sleep study and machine. You are typically on your own once you get it unfortunately, which is why the success rate of people using CPAP machines is usually pretty low. It can be a frustrating process without help and even if you do have good clinician, it is still frustrating to get used to it (some moreso than others).