Well, that depends on why she took it away.

You stored some notes on your phone and you need it for the test

You can't really convince your mom to give you your phone back. Instead, try being nice to her (even though you should anyway) and help out around, even a little more than usual. But don't keep asking for your phone or it seems like you're only being nice to get your phone back.

Play it safe for a few days, maybe weeks, then, when she hopefully forgets, take it back

I agree with @kayt68, it really depends on why she took it in the first place and how many warning you had before. If it was minor, offer to do extra chores ect, if you were given lots of chances and still screwed up, you may be out of luck

Like basically show her that you've learned whatever lesson she was trying to teach you by taking it. And try and be respectful, like many others here have said. I'd apologize for whatever it was as well.

Why did she take it away? Whatever you did to get her to take it away... Do the opposite of that. And in my experience, you can't really convince a parent to restore privileges, you have to prove that you are worthy of them.