I had a backup, so i have only left database and conf folders in place, removed all the new files (you can also leave resources/security, which holds SSL certificates) and copied old ones from backup. On Windows i have also tried just uninstalling and installing older one (on a test server). Usually it works.
I'm not using installation package on linux (just a tar.gz archive), so i'm not sure how it will behave when trying to install older version on top or if you just replace files.