To replace the o-ring washer on a mixer tap, you need to remove the retaining nut and fittings, lift the tap from the faucet, remove the o-ring and replace it with a new one. Then, reverse the steps to reassemble the tap.
Replacing the washer stops leaks around the base of the spout. Since the spout is past the valves, there is no need to shut off the water supply to the home to make this repair. Depending on the brand of mixer tap, removing the retaining nut sometimes reveals a snap ring or a setscrew that must be removed to release the spout.