Depending on the firmware/IOS of your AP.
Power up the AP (don't connect to the LAN) and in enable mode enter the command "clear lwapp ap controller ip address".
If the error message "Command disabled" comes up then you need to boot to the "rcv" image. This can be achieved by making sure you have a "rcv" image in the AP and deleting the LWAP IOS on the AP. Reload the AP and run the command again.