First of all, what I would suggest is update your firmware to the latest version available.
Next thing you want to do is go to Network > Interfaces while your WebUI has Advanced Mode enabled.
Once you're there, you're allowed to drag these interfaces as you like, interface on top will have the highest priority and priority will go down accordingly to the interface arrangement. (Another way to do it would be editing any interface and going into its advanced settings, there you can change Gateway metric too).
Another way to do it would be through CLI or SCP file manager like WinSCP. Go to /etc/config and edit file mwan3 with command vi
There you will find all the configurations of failover and priorities.