• Automated: use admin panel to upgrade webmail code (Standard edition),
• Manual: overwrite files on server with new files.
To upgrade the installation, you simply need to upload files from the new package overwriting existing files. Directory structure is organized to have each new version installed to a different directory. Only a few files will be actually overwritten (/index.php and /data/VERSION).
All the additional reconfiguration will be done by the product on next run.
Be sure to create a backup copy of the entire installation before proceeding with the upgrade. If anything goes wrong, you'll be able to restore previous install from backup.
Upon extracting the package, it's required to set permissions for files and directories.