So I have some mp4 files which had wrong metadata included in them (especially the title). How do I remove this information? What I tried was:
But it still shows the wrong titles. I would love not to have to rebuild the whole library because it is pretty big.
The trick is to tell Plex not to use embedded metadata as the first source of information:
Settings - Server - Agents - Movies - Freebase + TheMovieDatabase
drag down the line ‘Local Media Assets’ so it sits below the line(s) ‘Freebase’, ‘TheMovieDatabase’
Make the same adjustment under
Settings - Server - Agents - Shows - TheTVDB + TheMovieDatabase
Now go to the PrePlay page of one affected movie and press ‘Refresh’ on the left side.
Then wait at least 30 seconds.
If you see the title correcting itself, you can perform a Refresh All for the whole library later.
If there are some persistently wrong titles, check their edit page to see if their title is ‘Locked’.
If so, unlock it.
Then refresh the movie again.
Keep in mind that Plex can only update or refresh one thing at once. If you Triggered a Refresh of a whole library, you can’t at the same time go into a particular movie and refresh it. You must wait until the operation is finished.
For some very stubborn movies which got incorrectly matched before, due to wrong embedded metadata or incorrect file names, you may have to perform the Plex Dance.
I need local metadata to be the first source of information sometimes. But you still helped me. I did not clean the bundles. Now it worked 
Editing tags within Plex doesn’t edit the metadata embedded in the files themselves. You’ll need an editor that can edit tags in MP4 files, something like EasyTag or MP3Tag.
