If it's a factory original, you have to undo every single solitary screw on the center console (I'm assuming it's a center shift automatic.) The biggest problem is there is a strap that attaches to the back of the unit that bolts to the back of the radio and then, like an erector set piece, has about 12 holes in the strap, one of which will have a bolt attached to the back of the under dash. If you are just trying to get the old out so you can put a new one in, I would get a metal snip and just cut the strap as its not worth the hassle of unbolting the VERY hard to get at and awkward bolts on the strap.
Remember the antenna cable will also be on the right side bottom and there is usually very little give. If there is a factory wiring harness, do everything you can to try and save it intact as it will make installing the new one a lot easier. But after the strap in the back is off, the only thing holding the radio in will be these 2 tabs on the right and left sides that need to be held in (towards the radio) so that they release from their catches. It's a big pain if you don't have these two little 'y' shaped metal tools you can buy at a PepBoys that help loosen the side catches.