I have a leaky tank problem with my upstairs toilet. can I turn the water supply to the toilet? If so, how do I turn off the toilet water supply?
In almost all cases there should be a shutoff valve behind the toilet, just follow the visible pipe that comes out the bottom of the toilet tank. It will either be a globe valve that has a handle you need to turn many times to open/close, or a ball valve with a handle that you turn 1/4 turn to turn on/off.
If there is no shutoff valve, you would need to turn off the supply to your entire house. It would also be a good time to install a dedicated shutoff for the toilet.
On a toilet installed up tyo code here in the united states there is a valve located under the tank behind the toilet. Turn clockwise to turn off water to toilet. Flush toilet and be sure it does not fill the tank again just to verify and empty most of the water. Where is the leak coming from exactly? It could be leaking from several places