Best practice in general is to install Zimbra on a virtual machine and use LVM or another technique that supports snapshots.
This way you can create snapshots and in case anything goes wrong you just roll back to your snapshot.
Note: if you don't upgrade when this dialog appears but want to at a later time, please see additional instructions at the bottom of this post.
After selecting Upgrade, a dialog box will appear to show the download progress.
After the installation has completed, you will see the dialog below.
Within this dialog box, you will select the Exit Outlook & Upgrade button.
When Redtail performs email server upgrades, which includes upgrading to a newer version of our email platform, Zimbra Collaboration Suite, a new version of the Zimbra Connector for Outlook (ZCO) becomes available to our users.
If you already have a version of ZCO installed on your system, you will be asked if you want to upgrade your version of the ZCO to the latest version.
Snapshots can not only be used for having an additional and easy to use fallback point in case an update/upgrade goes wrong but also provide for easier backups: Have a complete backup of your whole /opt/zimbra folder and ALWAYS keep a copy of the Zimbra installer for the version and OS you use.
In case your server crashes you can restore your whole Zimbra installation with the folder and the installer.