Tap the check (✓) button on the episode. Tap it again to mark it unplayed.
When you mark an episode played, Castria also:
- takes it out of Up Next;
- deletes its download, unless you've saved it;
- skips ahead to the next episode, if it was the one playing.
To mark a whole list played (your Timeline, Saved, Downloads or a playlist), tap ⋯ at the top of the list and choose Mark all as played…. This also deletes those episodes' downloads (saved episodes keep theirs), and it can't be undone.
A show's own episode page has no "mark all" option. Mark its episodes one at a time.