Locate the cold water line that feeds your system. Turn the in-let valve off--1/4 turn so it is perpendicular to the water line. Open your kitchen faucet and drain all water out of the holding tank.
Unhook the unit containing the filters. Place a drain pan under the filters to catch spilled water.
Unscrew the first filter canister from its cap. This is the carbon-filter canister closest to the cold-water inlet. If you have difficulty unscrewing the canister by hand, use a rubberized mat (the type used to unscrew jars). If necessary, use a C-shaped filter cartridge wrench. Change the carbon filter about every six months.
Unscrew the second filter canister. This sediment filter should be changed about every six months.
Remove both the carbon and sediment filters from their canisters. Insert the new carbon filter in the first canister. Turn on the water. Catch the water as it flows out of the filters into your drip pan. Watch for the water to flow clear, indicating that the carbon filter has been flushed of tiny carbon particles.
Insert the new sediment filter into its cannister. Lubricate the "O" ring on top of both cannisters with a little silicon-based lubricant. Rub the lubricant on with your finger and tighten both canisters by hand.
Unscrew the third canister. This is the reverse-osmosis canister. Remove and replace the reverse-osmosis membrane. Lubricate the "O" ring with silicone-based lubricant and tighten the canister back in place. The reverse-osmosis membrane needs to be replaced approximately every 18 months.
Unscrew the small post-carbon filter. Replace the entire unit and tighten it on by hand. Place the filter unit back on its hook.
Turn the in-let valve back on, 1/4 turn so the valve is parallel with the cold water line.