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:

1. In the left sidebar: More Settings > Advanced
2. By clicking on the small grey “Advanced»” link you find in the top right of the device's web interface.

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

1. Connect only one cable from your PC to your router, and leave the other ports (LAN or WAN) unconnected.
2. Press and hold the Reset button firmly first, and then power on your device. Must obey the first and then procedure.
3. The LEDs will start blinking. Release your finger when you see the LEDs flashing 5 times for AR150 and 6416 ( flashing 3 times for MT300N and MT300A). Number of blinking to count for newer devices (tested on a BL-1300) seems to be 5, and after that it stops blinking.
4. Set your computer’s IP address to 192.168.1.2
   
5. User Firefox or Chrome to visit http://192.168.1.1. Then you will enter Uboot Web UI.
   The following screenshot is an example of the uboot web UI from a GL.iNet BL-1300
   
6. Read the instructions on the web UI, it may tell you that the file should have some word in the name to be accepted (the BL-1300 wanted either “lede” or “qsdk”, see the screenshot above). Find your firmware image, rename it if necessary and upload it.
7. Wait for around 3 minutes. Don’t power off your device while upgrading or you will have to repeat this procedure.

At the following link you can find the official page on the GL.iNet website with the instructions
https://docs.gl-inet.com/en/3/setup/mini_router/upgrade/