Firstly though, check online what adapter cable will fit the Alpine head unit to the car loom plug, then check what you have in your car with the Clarion head unit.
Make sure you remove the negative cable from the battery before messing with anything.
The Clarion HU will have needed an adaptor cable to fit to the car loom, and an adaptor for the aerial. If that is the case, you might be able to just simply unplug the Clarion, swap out the aerial adapter from the Clarion HU to the Alpine HU, and plug the cable adapter and aerial plug that was fitted to the Clarion HU to the Alpine HU.
I suggest getting a pair of head unit removal keys for the Clarion (you might be able to use the ones that come with the Alpine HU), a phillips head screwdriver, and a panel removal kit for the dashboard trim.
If you don't want to pony up for the panel removal tool, wrap a butter knife or flat-head screwdriver with electrical tape and carefully remove the trim with that.
After that, all you do is either just swap out the head units, or, if necessary, swap the cable adaptor as well and re-install. Re-connect the negative cable at the battery and you're done.