How do I change a tragus ear piecing?

Unscrew the ball at the front, and it should just come off. I think most of them you turn to the left to loosen. Once the ball is off, you can slip it out by pulling the flat bit at the back that's in your ear canal.
Then you take the ball/ front stud off your other jewellery, push the stick bit through your tragus from the back, and then screw the ball on the front.
It's quite fiddly, especially the first time. I'd have someone around to help you, as they're going to be able to get a better look than you. Good Luck