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Turn off the gas or electric to the hot water. For gas heaters turning it to Pilot or Vacation is sufficient.
Turn off the cold water to the heater.
Open the hot tap. Particularly the lowest and highest one in the system.
Drain about a bucket (1 gallon or so) of water from the hot water heater.
Turn off the cold under the sink.
Make SURE that no water comes out the tap your trying to change the shutoff valve to.
If it does, turn off the cold water main. Make sure the water is off before beginning work.