Before upgrading, you need to:
Download and unzip the new version to which you are about to upgrade.
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.
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.