Ask your doctors office if they will set it for you.
My DME will set it or my doctors office has people who will set it.
I can set mine myself because I watched and learned how, but I don't have a way to measure the pressure to prove it's correct.
My DME wouldn't tell me how to change it but it was easy to figure out after watching her do it once.
After setting it to 12 on the machine, her professional pressure gauge didn't read 12, it read 11.8. There is a fine tuning setting on my machine where she was able to bump it up 0.2 to make it read 12.
I bought a cheap gauge and the DME gave me another one that isn't that great either. Neither one reads exactly what her expensive gauge reads but they are both good enough to prove that the machine is continuing to work as advertised. In other words, I can put my cheap gauge on it and it reads 13 which is wrong but it reads 13 everytime I test it. If the machine stops putting out 12 for example and starts putting out a lower or higher pressure, my cheap gauge will report a lower or higher pressure.
For the techie's here: The gauge I bought uses a ball floating in the airstream. The gauge the DME gave me is a F&P tube with CM marks on it and a stopper to put it into the top of a humidifer exit port on a F&P machine. I cut two holes in the top of a plastic jar lid and put my hose in one and the stopper and tube in the other. Added water and turned on the machine. The water rose in the tube but didn't stop at 12, it stopped at 12.75.
Professional gauge: 12.00
Cheap gauge one: 13.00
Cheap gauge two: 12.75
Of course by science standards, the temperature of the water should be 32 degrees F. when measuring cmH2O. I guess you have to read it before it freezes