The process for restoring PPSSPP's settings is the same regardless of what platform you are using. This will open the PPSSPP settings menu. This will display the settings for the emulator system. You'll find it about half way down the menu. You'll be prompted to confirm. Your control mappings will not be reset, but everything else will be restored and the process cannot be undone. This will reset your graphics, sound, emulator, system, and networking settings to their default state. You'll need to exit PPSSPP and restart it for the settings to reset. Click "Back" or use the your device's Back button to return to the PPSSPP main menu. This will clear your settings and return them to their default state.

Summary:
Launch PPSSPP. Tap or click the "Settings" button in the menu on the right. Tap or click "System" at the bottom of the menu on the left. Scroll down to the "General" section. Tap or click "Restore PPSSPP's settings to default." Confirm that you want to restore the settings. Return to the main menu. Tap or click "Exit" and then relaunch PPSSPP.