Turning off water to change tap.

Have you checked for isolation valves underneath your tap? If non available then turn off your stop tap and valve on the flow from your hot water tank, run tap on hot until it stops running then your good to go. Or use a plumber 🫣

if your knowledge of plumbing is so poor, find a qualified plumber to do it for you and save yourself the aggravation and worrey.

Firstly check for local isolation if these are not available then depending on what hot water cylinder you have (vented/unvented) if the hot water is pressurised (unvented) simply switching off the main tap should isolate both posing no issues with refilling if your cylinder is gravity fed (vented) meaning fed by tanks in the loft you can look for a gate valve (red wheel headed valve)on the pipe that fills the cylinder from the bottom if it moves free close that and just have to drain the small amount of water in the pipe again no problem for refilling if not available or seized then it would mean switching off the mains and draining the header tank via the hot outlets this technique can throw up more issues the water inside the cylinder will remains so no issues for the boiler but when water is switched back on and the header tank refilled the possibility of an airlock can be high there are lots of ways these can be removed with plenty of youtube tutorials available a simple technique that works most time is covering the end of a mixer tap usually the kitchen tap and with the hot side fully open slowly open the mains cold side forcing the cold water back up the hot after a few minutes you may hear a gurgle then release and allow the hot to run the success of this method depends on the design of the tap and severity of the airlock hope this helps