Head to POS Devices
Hover over 'Settings' and click on 'POS Devices' from the menu options
It's important to note that you should only deallocate a device if it is not currently in use, if someone is using the device, it will prevent them from finishing any transactions or processes they are actioning and kick them off
Scroll down to the device you want to deallocate, ensure that under the Allocated column, it is set to yes.
Click the more button and select "Deallocate"