An InstaPure IF-20 water filter uses an IR-20 filtering cartridge that fits inside of the filter canister. The IR-20 cartridge is no longer being manufactured, and it has been replaced with a Pentek P-5 cartridge. They are available at hardware stores and many department stores, and will readily retrofit to the IF-20 filter canister. They have a rated capability of filtering 30,000 gallons of water, and should be changed at 6 month intervals.
Things You'll Need
Sink with a fresh water supply
Turn off the water supply to the filter canister. That can be done by either turning off a valve on top of the water filter canister, or turning off the main water supply in the house.
Attach a filter wrench to the canister and turn it counterclockwise to unscrew it.
Grasp the canister as it unscrews and detaches from the top portion of the filter. When it comes off, the actual filter inside will come off with it.
Pull out the old filter and discard it. Carefully remove the O-ring from the threads of the canister. Wipe it off with a rag to clean any dirt, debris or hard-water buildup, and place it off to the side.
Rinse out the canister at a different sink to remove any sediment or debris on the canister's bottom. When finished, carefully push the O-ring back onto the threads of the canister.
Place a new filter inside of the canister.
Liberally cover both the threads and the O-ring with petroleum jelly. That will ensure ease of assembly and a water-tight seal. Hand-screw the canister onto the tip portion of the filter. Tighten it securely with the filter wrench.
Turn the water back on and check the canister for leaks.