Usually, upgrading iRedMail is just updating some config files to achieve new features or fix bugs, you do NOT need to download the latest iRedMail and run the script.
Upgrades are only supported from one release to the release immediately following it. Do NOT skip releases.
If you're looking for upgrading a very old iRedMail server to the latest iRedMail release, there's an alternative solution: Migrating to a new iRedMail server.
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