How do I remove an incorrect tv show poster

It could be embedded into the video file itself.

Do this to avoid trouble caused by embedded metadata:
Go to

  • Settings - Server - Agents - Shows - TheTVDB

In there, grab the line ‘Local Media Assets’ with your mouse and drag it downwards, so it ends up being at the bottom of the stack of active agents.
Repeat the same under

  • Settings - Server - Agents - Shows - TheMovieDatabase
  • Settings - Server - Agents - Movies - Plex Movie
  • Settings - Server - Agents - Movies - TheMovieDatabase

Second possibility:
The movie could be incorrectly matched to a different movie with a similar title.
That’s why it is important to have the release year of the movie in parentheses after the official movie title. Movie Title (year).mp4

Look up your movie https://www.imdb.com/
determine the correct title and release year
(screenings at film festivals don’t count, only the official cinema launch)

Perform the Plex Dance to clear mismatches from the Plex database.