Upgrading firmware

So I upgraded the firmware on my ender 3v2 and now when I go to print the display monitor freezes and the printer won’t print.

How do I revert back to the old firmware version?

You put an alternative firmware (not necessarily the old one, but it could be) on the SD card, replacing the buggy firmware you had on it and reboot the machine.

It’s supposed to look for new firmware very early in the boot process. If it doesn’t find one on the SD card, it jumps to the firmware that has been installed - which in this case is buggy. If it does find one, it installs it and reboots. The point is, the boot-time-installer is not part of the firmware (on most systems) but is a simple, hard-coded section of code that should work no matter what firmware comes after it. Therefore it should be able to get you out of a jam, like the one you’re having.