Using the Oceanlog software -- OPen Oceanlog. Select "Dive Computer", then click on the "Alarm" tab. Select the Max PO2 you want. Connect Atom 2 via USB cable. Click on "Setting Options" on the toolbar, then click on "Save Setting to PDC." Click the blue arrow, "Start saving setting" to download settings to PDC. Disconnect all when finished and close Oceanlog.
On the PDC itself -- Hold the upper left button for 2 seconds to change to a dive mode. Then hold the bottom left and upper right buttons until SETA appears. Then press the bottom left button 8 times until PO2 appears. Press the upper right button to scroll through the possible values. When you have the one you want, press the bottom left button once more, SETA will appear. Press and hold the bottom left and upper right buttons until you return to dive mode.