Depends what wheel you have, but with mesh wheels which normally have bolts, just undo it all, get a plank of wood and a hammer, place wheels upside down so that you can hit the face out, remember to put them on a mat so you don't scratch the lip. Knock one side at a time. Keep going around so that it comes out evenly. You'll need to put a lot of force for it to come out. Also don't forget to release most of the pressure in your tyres out. Should help get the centre out.
Putting it back is a little more difficult. Everything has to EVEN. And I mean everything. hahah. If the face doesn't go in even, it's a pain in the arse for it to go in, if it's not even with the bolts, well you've got to hit it back out and start again. You'll get the hang of it. I had back pain and wanted to cry after finishing torquing up the one wheel's 20 bolts.
All I can say is have fun...