Synology Community
https://community.synology.com/enu/forum/5/post/12608905.03.2019 · Under normal circumstances, upgrading DSM does not affect the data on the Synology NAS. However, to ensure data security, it is recommended that you back up your data before starting the update. Once you update to the new DSM, you will not be able to downgrade DSM version. Responses (1-1) D Didier Hirt @deedoo Aug 15, 2021 4.
Release Notes for DSM | Synology Inc.
https://www.synology.com/en-global/releaseNote/DSMBefore updating, your Synology NAS should be running DSM 6.2 or above versions. Reserve extra time to complete DSM 7.0 update. The DSM update takes 10 to 20 minutes, while package update may take more than an hour. The actual time depends on the computing power of your model, the number of media indexing files, and the number of packages installed.