Re-scan the library, then empty trash (see pic below). Plex will pickup the new file name with the re-scan. Emptying Trash will remove old information from the server (it does not remove the file, it just deletes info from the Plex database).
FYI, since you’re new to Plex, you might review these two documents. They can help avoid common pitfalls when running Plex on a Synology NAS.
- Installation & Setup of Plex Media Server on Synology
- Synology FAQ - Questions, Answers, and a few How-To’s
