How do I replace a leaky toilet shutoff valve?

Turn off water. With two adjustable wrenches, one on valve and one on the nut behind it around the pipe, turn counter clockwise with wrench on valve and clockwise with wrench on nut.
Where is it leaking?

Well, that depends on the valve. Some are sweated in place. Some are pipe threads and some are compression fittings and some are even push on fittings. Examples here: Watts: Kwikstop™ Supply Stops - 1/4-Turn | Brass & Tubular

You have to describe the valve or take a picture.

Bellagirl... where is it leaking from? Is it leaking from near the handle... if so just need to hold the shutoff and tighten the packing nut as in the following picture. Go slow.. do not overtighten. Take care.

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