you goto halfords and get the removal tools (basically looks like 2 V shaped pieces of coathanger) which cost £2.99, insert them in the holes either side of the unit and pull it gently out of the dashboard.
Usually getting the wiring out of the unit is harder as the plugs seem to weld themselves together!
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix in-car audio.