Use the update path from the DSM release notes and backup first.
- DSM 6.2 or above is required for DSM 7.0 Preview.
- DSM upgrades are irreversible, so back up data first. Synology recommends Hyper Backup for data, system settings, and package configurations; Snapshot Replication is only for Btrfs destinations.
- For a manual update: DSM > Control Panel > Update & Restore > DSM Update > Manual DSM Update. Download the update file from Synology’s Download Center, upload it, confirm, and wait.
- After the update, the system restarts all services and packages.
What can break / need attention:
- Some packages may not be supported by Hyper Backup backup/restore. If they are not listed in DSM 6 or DSM 7, reinstall them on the destination NAS after migration.
- Packages cannot be restored onto a destination NAS running an earlier DSM version than the source NAS.
- Time Backup and HiDrive Backup are not supported in DSM 6.1 and above.
- Manual re-indexing is required for all files after upgrading to DSM 6.1.
- Packages may need re-initialization after upgrading to DSM 6.1, and this can take up to an hour depending on the number of packages installed.
- DSM 7.2 notes that after a DSM update, you cannot downgrade to previous versions.
If you meant migrating to another NAS rather than just upgrading the same NAS:
- With Migration Assistant, the destination DSM must be the same as or newer than the source DSM.
- With HDD migration, the destination retains the newer DSM version, and the destination NAS must support that DSM version and file system.
- With Hyper Backup, the destination DSM must be the same as or newer than the source DSM.
Sources used: [1], [2], [4], [10], [13]