FortiAP devices are thin wireless access points (AP) supporting the latest Wi-Fi technologies (multi-user MIMO 802.11ac Wave 1 and Wave 2, 4x4), as well as 802.11n, 802.11AX , and the demand for plug and play deployment.
Article Id 198557
This article describes how to change out of box FAP-U boot image.

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FAP-U are compatible with FortiWLC enterprise wireless LAN controllers, integrated FortiGate enterprise firewall LAN controllers, and FortiAPCloud management platform.
If FortiWLC controller is discovered, the unit continues booting up with the default FortiWLC image.

If the FortiGate controller or the FortiAPCloud management platform is discovered, the unit reboots with the FortiAP image.

Follow below steps to change the default boot image from FortiWLC to FortiGate:
1) Both images will be running on Universal FortiAPs.
2) Connect the Laptop to the FortiAP Ethernet interface directly using a cross over cable. If the FortiAP is powered on using the PoE switch then connect the FortiAP and the laptop to an isolated switch or hub. Make sure that the FortiAP does not get any IP address from the DHCP server.
3) Default IP address of the FAP is
4) Configure the laptop with a static IP address within same range 192.168.1.x/24 (for eg:
5) Access the GUI of the FortiAP using the IP address from the laptop browser.
6) The following screen will appear.

7) Image ID signifies default boot image.
Meru Secondary: image used to connect the FortiAP to the FortiWLC.
Fortinet Primary: image used to connect the FortiAP to FortiGate or FortiAP Cloud.

8) Select 'Configure' button to change the Image ID.

9) Select the required Image ID using the drop down list and select 'Apply'.
In this example the Image ID has been changed from Meru Secondary to Fortinet Primary.
Check the screen below.

10) Select 'OK' and the FortiAP will reboot using Fortinet image.
11) Close the browser and again try to access the FortiAPs GUI using its default IP address
12) The login page of FortiAP will appear running Fortinet image.

13) The configuration page will appear on the FAP-U once the login is done with using the default username admin and no password.