Change the default resolution of the Ubuntu Server 20.04

I’m using RockPi 4B with Ubuntu Server 20.04 which is downloaded on the official ROCK Pi 4 website.
I want to change the default resolution of this system, the default 19201080, but I want to change it to 1024600 for my 7inch LCD. Because the fonts is too small on the 7inch LCD.
Is any one know where I can change it? I searched the method on the website, they said it need to change on the /etc/default/grub. But I cannot find this file on this system!