CAPWAP APs only receive software updates via the controller. That includes receiving an updated image that turns them back into unmanaged stand-alone units.

Here are the steps to re-image an AP:

1. Power the AP and wait until it joins a wireless controller. That may take a while, especially if it was originally joined to another controller. Verify it joined using the controller web GUI, or using the CLI with the command:

show ap join stats summary all

Make sure you get the name.

2. From the controller CLI, issue the command:

config ap tftp-downgrade <TFTP-Server-IP> <Standalone-IOS-Image> <AP-Name>

If you have a console connection to AP, you can see it write files from the image tarfile to flash and then reboot.