If you’d like to transfer the adapter to another car, first make sure that it's listed as supported on our Car Compatibility Page.
After that, open the Automatic app and select the launcher on the top left. Look for this icon: 
Tap on "ADD A VEHICLE" near the bottom and follow the in-app instructions from there. See below for a screenshot.
Please note that these steps must be followed each time you move the adapter from one car to another. The adapter will not know which car it is installed in otherwise.
If you need to remove a car, please see this FAQ.