Mark seasons watched from the show summary
On each TV series page, there are posters for each season. These posters should have a hover option for marking them "seen".
You can now click the check mark to add any season as seen.
Daniel Hawkins commented
Also, this should be a toggle (mark "seen" or "unseen") -- it's frustrating that on season pages you can only mark as "seen" but cannot mark as "unseen" if you made a mistake or wish to track your progress through a second viewing of the series.