Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
Generally speaking, with dash-mounted device (though it may depend a lot upon what car or truck you have it in!) you get down on the floor with the door open, and look up under the dash devices, undo a fastener or two there, and push the device(s) out of the dash from behind. You may also have to undo wire connectors, perhaps - if you do, note them down for sure and without mistake! as to how they go back.
Let me know how it goes, or if you need other advice.