All GL-inet devices share the same general installation instructions as they all have a firmware based on OpenWrt, and all have a uboot bootloader with web interface to upload a new firmware.
You can access LuCi (the normal OpenWrt web interface) by clicking on the small grey “Advanced»” link you find in the top right of the device's web interface.
From there you can upload and install a new OpenWrt firmware as normal.
This applies to all GLI routers, including 6416, AR150, AR300, MT300A, MT300N, MT750, Domino boards and newer products.
This method can be used also as a recovery system if you brick the device.
According to the link provided in the upgrade web page, it is based off Pepe2k's Uboot mod
At the following links you can find the official page on the GL.iNet website with the instructions