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Before the upgrade, you need to:
Back up data, as described in Backup and Restore, before upgrading. In case anything turns badly, your data is safe and you can start over.
Back up customizations (including configuration, deployed extensions and applications) that you plan to reuse in the new version.
Download eXo Platform 4.4 version.
Perform one or more dry-run upgrade(s) to find out potential problems and estimate the upgrade time.
The dry-run upgrade allows you to:
Detect and handle issues to make sure they will not happen during the real upgrade.
Estimate how long the upgrade will take in your production environment.
Find out if you need to adjust anything to make your upgrade faster and more efficient.
With eXo Platform 4.4, Tomcat was upgraded from 7.0.63 to 7.0.72 version and JBOSS from 6.2 to 6.4 version to benefit from their last updates and improvements.
You can check changelogs for tomcat and for JBOSS.