Ok sorry - not sure why you would want to do this unless you plan to connect it to a controller or cloud though.
Through your support account you can download the standalone firmware for your AP. In the admin section to Unleashed where you can 'upgrade' the device use the standalone code you downloaded. When it reboots it will be back in to standalone.
Note that you have to get past the Unleashed wizard first so just set up anything, in order to be then able to log back in to the UI and complete the process mentioned.
You can also upgrade the firmware via CLI - but I'd recommend to use GUI unless you have some challenges as it is simpler.