Turn off the water to the house, and remove the screw on the knob. Then remove the mixing cartridge with a wrench. Take the old one to the hardware store and match it against the new one to be certain it's the correct one. Then reverse the steps.
If that doesn't fix it, then the whole valve may need replacing. That's costly and messy since they have to open up the wall.
If that doesn't fix it, then the whole valve may need replacing. That's costly and messy since they have to open up the wall.
