The stop valve is one of three types: soldered on, screwed in, or compression fitted. Any one of them can be replaced.
A screwed in one is basically just unscrewed.
A soldered in one would need to be heated up with a torch and pulled off.
A compression one has a nut on the back part of it. Undo that nut, and then pull the valve off. If the nut is not in too bad condition, you can often reuse it, otherwise, you have to pull it and the ferrule off. They make a special tool to do that if it won't come off easily (probably won't).
Usually, the valves have a washer in them. You can usually replace it, but the seat may not be smooth and a new washer may not seal either.
A new valve is usually less than $5, so they aren't a big investment if you can replace it yourself. While you are at it, replace the hose or pipe from the valve to the toilet. The solid pipe may look better to some people, but the reinforced hoses are much easier to install. Hope this helps.