Yes! You just need to download the correct Zimbra Network Edition installation software, www.zimbra.com/downloads/ne-downloads.html and then perform the installation on your existing Zimbra server.
No, the upgrade process requires downtime for the Zimbra server.
No, the Zimbra webmail client will automatically prompt users to reload their Zimbra webmail after an upgrade.
To upgrade your Zimbra Network Editor OR Open Source Edition server: Download the Zimbra installation file to your server For Zimbra Network Edition, www.zimbra.com/downloads/ne-downloads.html For Zimbra Open Source, www.zimbra.com/downloads/os-downloads.html Unpack the tarball, tar -xzvf
This is highly dependent on the number of mailboxes and Zimbra servers in your organisation as well as any customisation that may have been performed. For a single Zimbra server in an organisation for 25 to 900 mailboxes the upgrade process takes 30 to 45 minutes.
Zimbra upgrades are released every 2 to 3 months and include bug fixes as well as new features. Patches are released for critical security issues, as required. The release schedule for Zimbra is available in the Zimbra Product Management Portal and provides details of all bugs, features and the corresponding release schedule.
Schedule downtime for the Zimbra server. Perform a complete backup of the Zimbra server data. Download the latest Zimbra upgrade. Install the upgrade over the existing installation. Check results. The actual Zimbra upgrade process itself can take 30 to 45 minutes or more depending on the size of the installation, number of servers, complexity etc.
New features, bug fixes, integrated software upgrades (eg. ClamAV upgrade to latest version) and security fixes. For critical security fixes a patch is released sooner.
Absolutely, see the Zimbra roadmap and for more detailed information, try the Zimbra Product Portal which contains the schedule for new features, bug fixes and feature requests.