Differences between updates and upgrades An update is a new distribution image that contains cumulative fixes and enhancements to the product since Pega 7.1.0; it is not a full product release. In contrast, upgrades are full product releases.

If you need to move from any version prior to Pega 7.1.0, you must upgrade. To move from any Pega 7.1.x version to the most current release, run the update. For more information, see the Pega 7 Platform Upgrade Guide. 

Reference links - 

https://pdn.pega.com/documents/pega-722-platform-update-guide

https://pdn.pega.com/documents/platform-support-guide