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 use the GL.Inet web interface to re-flash your device using OpenWrt (Upgrade > Local Upgrade).
You can also access LuCi (the normal OpenWrt web interface) and from there you can upload and install a new OpenWrt firmware as normal. Depending on the web interface version, you can access LuCi:
This applies to all GL.inet 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 link you can find the official page on the GL.iNet website with the instructions