For me, firstly I would like to download the burning tool and upgraded by USB-C: link
the method descriped on webpage is very easy to learn.
I am not a technology man but I can do it, so you will
Power key is the far-est key to USB slot.
Reset key is the nearest key to USB slot.
download the newest images of ubuntu, you need ubuntu Mate or Ubuntu Server?
maybe you can check on the firmware page of Khadas VIM link
after installing the burning tool (if re-named the file, dont use special symbol)
check step by step on the first link. you will find it is a very easy way.
Notice: the first time of power on after upgraded need to take 3-5 minutes to wake up