Downgrading firmware is tricky nowadays and impossible without root access. You should treat firmware upgrade as a one-way ticket. If your currently installed firmware version has no known issues, you should not upgrade,
or you may be stuck with a buggy firmware version.
If you really want to upgrade, I suggest upgrading one version at a time until you are happy
with it. Research the firmware version you plan to upgrade to online (Reddit, Discord) to see
whether it contains any known issues that affect your usage and grab the firmware upgrade
files from my archive.
There is no firmware-wide anti-rollback mechanism, you just need a special tool to bypass the
checks. Pick the tool that matches your webOS version. The projects themselves have detailed
instructions on how to use them: