If this is a test server then what you need to do is the following:
That should get you back to a working system of your previous version.
1. Remove the current ZCS 6 with ./install.sh -u - that will remove everything.
2. Next, run the installer (using the same version of Zimbra as your backup) with the -s option: ./install.sh -s
3. Remove the newly created directory: rm -rf /opt/zimbra
4. Restore your backed-up directory
5. As root run the /opt/zimbra/libexeczmfixperms utility
6. When that's done run the installer again: ./install.sh
It gets the package information from your package database.