The version you have is the Open Source version (FOSS). Download the lates release of Zimba and stop Zimbra,
take a backup of the /opt/zimbra directory and while that's running read the Release Notes carefully. After the backup has finished and if there's nothing in the Release Notes that you need to do then just unpack the downloaded file run the ./install.sh script. I'd also suggest you search the forums for any other problems that may have cropped up for other users.