Instructions
Hold the PSP power button until the PSP turns off. Hold the "Right" trigger button and turn the PSP back on. This enters recovery mode.
Select "Registry Hacks" from the recovery menu. Select the first option, "Button Assign." Selecting "Button Assign" once changes the X button to the O button. Selecting twice changes it back.
Select the "Back" option at the top of the "Registry Hacks" menu and select "Exit." This reboots the PSP. Do not hold the "Right" trigger or it will boot back into the "Recovery Menu."
Select the "Settings" icon using the "O" button to make sure it is now being used as the selection button and not the back button.