Changing a washer on a hot tap

[JCS1965]

Do I have to drain the hot water tank to do this or is there another way ?

[gasman]

There Should be a gate valve in the airing cupboard it will have a red or green wheel type handle on it, or a fullbore lever type valve this should be attached high up to the fat pipe that goes into the bottom side of you're hot water cylinder. This should be turned in a clockwise direction until it is closed and will not turn any more This will isolate the hot water without the need for a full drain down.

[JimmyK]

It depends whether you have a hot tank or not. the method described by gasman is if you do.
if you have a combi-boiler ie no hot tank you have to turn off the cold water into the house