On an older house, they may have used a solid riser. The valve could also be threaded onto a threaded nipple (short pipe threaded on each end).
An old valve like that is too much trouble to fix, so it is usually just replaced. If it is threaded on, you risk unscrewing parts in the wall if you take it off. this isn't necessarily catastrophic, but the problem is, you can't see what's going on. If it is soldered on, without tools and a little experience, that's a little risky, too as a first project.
If it is just the riser pipe that goes from the shutoff valve to the toilet, that can be removed and replaced with a flexible hose designed for that. You want one that looks like braided stainless steel long enough so it isn't pulled tight when making the connection. That, you should be able to do yourself. Problem is, you'll have to shut the house water supply off since your valve is frozen.
If it is threaded on and you feel lucky, shut the water off, and unscrew the valve after disconnecting the riser pipe to the toilet. If it unscrews and the pipe in the wall doesn't, then it's pretty straightforward to screwing on a new one after coating the threads with pipe dope. if the pipe in the wall comes loose, you might be able to get it reinserted (after putting pipe dope on the threads), but it's sort of like threading a needle in the dark. The only good thing is that it's probably straight back there. You won't be able to see if that junction leaks, though, so it is risky. You might have to break open the wall to reassemble it which is why I said it was risky. Now, if it is also galvanized steel, it could be corroded and fall apart, too.