How should I clean a filter cartridge instead of replacing it?

Soak it, don’t blast it. Use a garden hose with a gentle nozzle, not a pressure washer. Spray between the pleats from both sides until the water runs fairly clear, then soak the cartridge in a filter cleaner solution for a few hours. Rinse again and let it dry completely in the shade before putting it back. Most cartridges last 3–5 seasons if you clean them properly.