How can I manually clean the print head on my Canon MP237 to resolve Error 5100?

You can try the Power Cleaning method. Turn off the printer, unplug it from the power source, then press and hold the Stop button while plugging the printer back in. Continue holding the Stop button until the printer displays a message. This initiates a more aggressive cleaning cycle. Repeat this process 2-3 times. Alternatively, soak the print head (carefully removed if possible - see online guides) in warm distilled water for 30 minutes, then gently blot it dry.