The car radio cassette player was the pinnacle of car audio for many years. Cassettes were an innovation in car audio---they were small, reliable, and provided good-quality sound. In addition, they were immune to the issues of skipping or cross-talk that had plagued their predecessor, the 8-track tape. Now the cassette player has been superseded by the car CD player, and many owners of older cars wish to upgrade their stereo. The first step in that process is removing the car's cassette player.
Remove any media you want to save from the car cassette player. Turn off power to the car at the ignition by removing the key.
Examine the dashboard of the car. Look for trim panels that surround the radio location. In some cases these are fairly small, and only surround the radio, and in some cases they surround the radio and many of the instruments and climate controls.
Look for any screws or bolts holding the trim panel on. Check especially carefully in areas that may be out of sight, like just underneath the dashboard. If you find any screws or bolts attaching the trim panel, remove them and put them aside. It's useful to label the location they came from or take a digital picture of the spot so reassembly will be easier.
Use the panel tool to pry off the trim panel. Most trim panels have several clips holding the panel on---they will release when you apply pressure with the tool. Work your way entirely around the panel, then pull it away. If the trim panel has climate controls built in, unplug the connectors on the back and remove the panel. Again, keep a record of your work.
Undo the screws/bolts holding the old radio cassette player in place. These will have been exposed by the removal of the trim panel. Pull the radio out and unplug any connectors on the back.