These pics are for removing a nav/cd unit but it should be pretty similar to a non-nav unit.
1 - Remove the radio trim piece. It's just held in by clips and it pops out with gentle pressure, starting from the top along the sides. Don't pull it all the way out, though. There are 3 cables holding it in place.
The red arrows show where the clips are; the yellow arrows show where the cables are.
Disconnect the cables and remove the trim.
2 - The radio is held in place by 4 phillips screws (indicated by red arrows here). Remove the screws and pull the radio out. There are (of course) cables on the back of the radio that need to be disconnected to remove it entirely.