Press the power on button, let the unit blow for a couple seconds, then press the power button again. The blower will shut off.
Press the farthermost button on the left (the "Ramp" button") and the left-arrow button right beside it, simultaneously, and hold them both down (around 5 seconds) until "P - 1" shows up on the display. This will indicate you are in the Setup Mode.
At this point, you will be allowed to cycle through the setup menu. As you depress either the left or right arrow key, you will land on several icons on the display that indicate which setting you can adjust. You want to get to the "cm H2O" icon. Depress either left or right until you reach that icon.
Press the power button once, and your pressure setting will blink. You may then use the left or right buttons to change your pressure setting.
When you are done, simply press "Ramp" and the left-arrow button simultaneously, and hold until "P - 0" flashes briefly. You are then back in the standard operating mode.
(This procedure has been written whilst I was working side-by-side with the machine. Sorry for the initial complexity, but I wanted to relay it in the most approachable way for everyone.)
I went to the homecare provider today, and brought my unit in for them to examine. While I was there, I managed to "eavesdrop" on the clinician's finger presses, and surreptitiously learned the key-press sequence required to access the Setup Menu.
Please, note that the left and right arrow keys stick. You must be SURE you are pressing the left-arrow key independently. I found this was easiest to achieve by using my index finger to depress the "Ramp" button, and my pointer finger to depress the leftmost edge of the left-arrow button.
I hope this helps others.