You need special tools for that. You can probably get them at the local part store. I think the Autozones around me sell them. They look like two big metal "U's". They plug into the holes on the sides of your radio. There is a hole on one side and one right below that and the same on the other side. Put the U tools in the holes and they pry the holding pins out of the way so you can remove the radio.
if you want to install the HU with little problems you should buy a wiring harness from walmart for 6.94. i just bought a cd player for my car and i tried putting it in, but i wasnt getting any power to a wire so im taking to tweeter. if you get the wiring harness it will make your life alot easier and you wont have to cut into any wires. the cd player should have come with its own wiring harness which should have about 12-14 wires coming from it, those wires connect to the harness you buy from walmart and from there the set up is that simple. but if you run into any problems i wouldnt try and fix it. id just take it to a pro.