How do I fix a leaky turn-off valve for the toilet ?

Are you a plumber? If not, are you comfortable sweating pipe?
I ask, because depending on how much pipe is stubbed through the wall, that may be the only way.
IF you are comfortable sweating pipe (and I expect you're not, or you'd likely not be asking this question) then you'll need to turn off the water to the house first.
Then, hit the valve where it is soldered to the pipe with the torch until it comes loose. Sweat the new one on, turn back on the water. Bang, you're done.

IF there's more than an inch or so of pipe protruding through the wall, you may be able to cut off the existing valve and put on a compression-fitting valve. Not the best way, but it works.

Hello sivkatarina:

Where is this shutoff leaking from.. I wonder if it is leaking from the PACKING NUT of the shutoff.. If it is the packing nut should only need to tighten the packing nut a 1/4 to a 1/2 turn to stop the leak (see pic. Below for idea on what I mean). Here you would hold valve in one hand and hold against yourself as you tighten the packing nut (clockwise) with a pair of pliers or a crescent wrench (as in pic.). Just do not overtighten and this should fix packing nut leak.

If leaking from the COMPRESSSION FITTING on shut off valve (where riser attaches to shutoff... again, should only need to hold against yourself and tighten that a 1/4 to a 1/2 a turn clockwise... again, do not overtighten.

From there... I would need more information to help you decide the best plan of action. Get back to me and let me know your thoughts about what I have said... talk soon... MARK.

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How do I fix a leaky turn-off valve for the toilet ?
You have two places for your valve to leak from. The bonnet packing nut, The one on the stem and the compression nut, the one on the stub-out.
Both are easy fixes. If it's leaking from around the stem then open the valve until it starts to leak around the stem. Now take a crescent wrench and began to snug the bonnet packing nut up. STOP! When it stops leaking.Do not over tighten. If the compression nut's leaking place a wrench on the valve body so the valve doesn't turn and crank down on the nut until the leak stops. Good luck and let us know how it went. Regards, Tom