Establish whether the library item is old or new
Consider a local film already listed in Kodi whose synopsis you corrected in its NFO. The video still has the same filename and path. Running Update Library searches for new material; the video-management manual says this does not import changed NFO information for an existing title. Check the path and filename first, because a moved or renamed video belongs to a different scanning case.
Try one title before a whole source
Kodi documents an individual Refresh from the title’s Information page. Open the intended entry, choose Refresh and read the prompts concerning existing local information. Work on one recognisable title first and compare the specific corrected field afterwards. A source-wide change can touch many entries and is unnecessary when only one local record needs correction. Keep a copy of your NFO before altering it further.
Check which metadata file is actually being read
The movie NFO manual describes precedence and naming requirements, including the distinction between a filename-matched NFO and movie.nfo. If an older local file remains authoritative, repeatedly refreshing will not select the edit you intended. Inspect the applicable naming rule and the selected content settings. Once the expected text appears, verify that the original video still opens normally. This is a metadata-maintenance task; it does not change the film’s playable content or establish a new viewing entitlement.
Questions, answered
Should I rescan an entire collection for one corrected synopsis?
Start with the documented individual Refresh and the actual NFO naming rules; a broad source refresh may be unnecessary.