It is perfectly right to do so. Read the release notes of 6.0.13 and follow the standard procedure for upgrading.
1. Always good to have a full backup of accounts, and your /opt/zimbra directory.
2. Make note of the services you have been running on the server. (zmcontrol status)
3. Stop your zimbra server with 'zmcontrol stop' (if it is a multiserver setup, stop your mailstore servers first, and then the ldap server. Upgrade ldap server first, and then the mailstores. Upgrade and keep ldap running when working on mailstores)
4. Make sure no zimbra process is running
5. Run the installer script as root user. (I always do this on screen)
6. Follow the instructions.
7. After the upgrade, update your forum profile too
