In the FAQ is the migration procedure
There are two Scenarios ( 1 & 2 ). Scenario 2 probably applies to you.
Given you have a balled-up DSM 7 installation
-and-
Your data is still in the “Plex” shared folder
-then-
- Uninstall the DSM 7 app with the ERASE option (gets rid of the bad DSM 7 install)
- Download the appropriate package from https://plex.tv/downloads
- Follow the procedure in the FAQ.
– BE ABSOLUTELY CERTAIN to click “Apply to this folder, sub-folders and files” when granting FULL CONTROL permissions
Migration takes time. ( about 3 minutes for each 1000 media items you have indexed ).
Be patient and wait.
Use a 2nd session tab to the Syno if you want to monitor its progress