Prepare for valve replacement.
Replace water shut off valve under bathroom sink.
Then open a faucet on the lowest level of your house and another faucet on an upper level to drain the pipes.
Deburr the pipe with a deburring tool.
There are two basic types straight stops and angle stops and these valves normally remain on and are seldom closed functioning maintenance free for years.
Shut off the circuit breakers to an electric water heater.
Open the faucet to drain the line.
When cutting the supply line cut slowly and gently so.
Place a bucket under the old valve.
If you have a gas water heater turn the knob to the pilot position.
Cut the supply line with a pipe cutter leaving enough room between the escutcheon plate and the cut to allow installation of the valve fitting.
If this happens to you you can simply replace the valve with a new valve that matches the old one.
Turn off the next valve up the line which may be the main water supply valve.
Shut off the water at the main shutoff valve.