Without forcing a complete refresh for the library with the Cogwheel, the easiest way to refresh a single series is to:
1) Move the series folder out of your TV Shows folder
2) Update the library ( just the little circle with an arrow in the top right)
3) Empty the trash (Removes entries that are no longer found such as your recently moved TV Show)
4) Clean the bundles (Removes the stored metadata from the entries removed in step 3)
5) Put your TV Show back in the TV Shows folder
6) Update the library ( you should see plex readd the item and attempt a clean match for the item)
The other option is to attempt to 'Fix incorrect match', To do this when you are inside the show (The screen where you can see the seasons) click the 3 dots on the left side('. . .') and select fix incorrect match. You should have the series popup in a list where you can reselect it.
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Edited to reflect BigWheel's corrections
