1) Replace a whole house carbon water filter cartridge whenever the water flow slows down in your house or after one year of use, whichever occurs first. You definitely don't want bacteria growing inside that filter! Prepare a a bucket with clean soapy water ahead of time because once you open the filter housing you probably won't have access to water until you're finished.
2) Turn your water supply off. You probably have a shutoff on the supply line located before the filter. If not, go to the hydro panel and turn the pump off.
3) Relieve the water pressure on the lines before you unscrew the housing that holds the water filter. You can turn a faucet on inside the house to relieve pressure and let the water run until it stops. Then shut off the water after the filter housing to keep the water from the house from draining back.
4) Place a bucket under the filter housing. If the housing has a relief button push that to make sure all the pressure is gone. Unscrew the housing and take it off.
5) Remove the old filter and clean the inside of the housing with soapy water and towels to remove any built-up debris. Put some clear plumbers silicone grease on the O-ring, which will help preserve the O-ring, make it seal better and make it easier to remove the next time. Unwrap and put the new filter in the housing and screw the housing back in place, slightly more than hand tight.
6) Turn the faucet off that you had running to relieve pressure. Turn the main water supply back on, slowly. Check for leaks around the cartridge housing.
7) Flush the new filter with at least 3 or 4 gallons of water or until the water flowing after the filter is clear.