You can upgrade uboot via uboot web page. You have to find a Windows machine to do this.
Upgrade uboot is the same step with upgrade firmware. Please download the latest uboot from https://github.com/gl-inet/uboot-gl-ar750-ar750s-x750/raw/master/bin/uboot-gl-ar750s-20181008-md5-3ca5683e0e40be42842051481bd072c6.bin.
- Connect your PC to the router’s LAN port, other ports are idle
- Hold on the reset button and power up the device until the middle LED is lighted
- Open Chrome browser, enter http://192.168.1.1/uboot.html to access upgrade uboot page
- Upload the uboot binary file and upgrade it, wait for the device boot up
You can check the uboot version after the device boot up.
root@GL-AR750S:~# strings /dev/mtd0 | grep -nr "U-Boot" | head -n 1
348:U-Boot 1.1.4-gcf378d80-dirty (Aug 16 2018 - 07:51:15)