if you need to remove the whole stereo surround, theres 2 screws just above the heater controls, remove those, and then gently, from the bottom, pull the whole facia off. Its clipped in at the bottom, mid way and the vents in the top. You may be able to remove the radio out easier once you have the whole unit off.
Or if you dont want to pull ya car apart, check the manufacturer details, pop into Halfords or one of your local car audio stores and get the correct removal tool. Most are about £2 - £3 and make life VERY easy!!
Depending on the new stereo that you are installing you may also need a harness adapter too to plug it in and possibly an aerial adapter too.
the stereo thats in at the moment doesnt have any place for the normal removal tools to go i.e. there are no sockets on the front to put the removal keys in. So im gona have to take the whole lot appart.
The correct removal tool is a phillips screwdriver as the stereos are bolted in place!!!.
As said before remove the facia and undo the four screws holding the two brackets and the stereo along with brackets comes out, i reused these brackets when i fitted my alpine and makes it a good solid fitting and near on imposible to steal in a hurry!!