9.1 3 does not support Standard downgrading to an older version. However it is possible to roll back to the previous version by unpacking the old MZ_HOME backup that was done earlier.
It is therefore recommended that you perform a Simplified Upgrade Procedure to ensure that the existing environment is present to fall back to in case the upgrade to version 9.1 3 fails. The steps detailed in Simplified Downgrade Procedure will guide you to restore the previous environment after a simplified upgrade procedure.