The mesh aerator on sink faucets serves a couple of purposes. The screen mesh in the aerator mixes oxygen with the water to maintain an even flow while not reducing pressure. The mesh also filters out large bits of sediment that are in your home water lines. Although it is possible to clean a mesh aerator, eventually it becomes time to replace it with a new one. Replacement mesh aerators are available at most hardware stores and home improvement centers. It is necessary to remove the old aerator first, to ensure you purchase the right size for your faucet.
Grab the outside of the existing aerator with a pair of slip-joint pliers, and turn the aerator counterclockwise to remove it from the faucet. It is not necessary to protect the finish of the aerator since you are replacing it with a new one. If you have an aerator that threads into the faucet rather than around the spout, it is still possible to grab enough of the aerator to turn it with the slip-joint pliers.
Take the old aerator to a home improvement store. Look for a replacement mesh aerator that matches the old one. The threads should be the same, either outside or inside. Find one that is close to the same height as the previous aerator.
Clean the threads on the faucet spout with a rag to remove any sediment. Push down the parts inside the new mesh aerator. Inspect the rubber washer at the top of the aerator to ensure it is not twisted.
Thread the new mesh aerator onto the faucet until it is hand tight. Turn on the faucet and check for leaks between the spout and top of the aerator. If necessary, place a rag around the aerator and tighten it with the slip-joint pliers.