sounds kinda like the one i have...
first you must pry off the black rectangle trim that borders the CD player. if you look closely, there are two tabs, one on each side of the head unit. the tabs are spring loaded and lock into place. if you push them both up at the same time, you can slide the head unit out relatively easy. It is just those two spring loaded tabs holding the head unit in the bracket. push the tabs up and it slides out ... easy!
if this isnt your cd player, then im not sure off the top of my head, but i imagine it is similar