Undersink water filters are basic, straightforward devices that need only a periodic cartridge change. Here's how you do it:
Turn off the inlet water using the blue inlet valve that comes with the filter (or the main cold water valve). Open the water filter ledge faucet and lock it open. When no water comes out, you're ready to service the unit.
Place a towel or a pan under the unit to catch spilled water. Open each of the vertical filter vessels by unscrewing the sump (bottom part) with the filter wrench. Remove the old filter cartridges and discard. If the cartridges are axial style (with a washer on top only), note which end was up on the old filter so you can point the new one in the same direction.
Lubricate the black o-rings very lightly with silicone lube — Vaseline will work, but silicone is better.
Install the new cartridges. If there's a sediment filter, it goes first. After that, it doesn't matter a lot in which order the cartridges are placed, but it's usually best to place a carbon filter last.
Carbon blocks and sediment cartridges are radial flow cartridges, so there's no “up” or “down”. With axial cartridges — cartridges with hard plastic bodies and a washer on top — place the washer end up. (If you put an axial cartridge in upside down, you'll discover it quickly because no water will come through the unit.)
Return the filter sumps with their new cartridges onto the unit. Tighten with the wrench until they're snug, but don't over tighten.
With the ledge faucet still open, open the inlet valve and let water into the unit. It will hiss and sputter for a while until air is expelled from the cartridges; this may take several minutes. Leave the faucet open and let the unit run to drain for 5 minutes or so, then close the faucet.
The unit is now back in service. Sometimes the water will appear milky for a day or so after the change. No problem. It's just air trapped in the unit that has to work its way out. The water is ready to drink.
(whole house & well units)