how to upgrade my mac catalina to big sur

Big Sur requires one of these…

MacBook (2015 or later)

MacBook Air (2013 or later)

MacBook Pro (Late 2013 or later)

Mac mini (2014 or later)

iMac (2014 or later)

iMac Pro (2017 or later)

Mac Pro (2013 or later)

FWIW, both versions are outdated.

If your Mac can leverage a newer version of the OS, it can gain additional years of productive use.

So yes, imho, it is worth upgrading.

Upgrade the computer until the upgrades stop, then start planning to replace it.

What Mac do you have? (See  > About This Mac). The model and hardware model year will determine whether you can upgrade from Catalina – and if so, how far.

Update macOS on Mac - Apple Support