Can you mar episodes as unwatched on Apple TV using iPad or iPhone?

I'm guessing you want to remove the checkmarks that show up on each episode that indicates that you have watched the episode.

If that is the case I need more info as it works differently if the series is an Apple TV+ show or one from an installed app that integrates with the Apple TV app.

What is the series you did this too?

On Apple TV Plus if you open the video to play, there’s an icon right beside the X on the top left of screen you tap that and it will remove the check mark. To do the opposite to mark as “watched” , you can do it on the episode thumbnail by long pressing to bring up the selection.

This can depend on the iOS version you’re using as well

This is what works for me for services that aren't fully integrated. I will start playing the episode from the Apple TV app. As soon as the episode starts to play, stop it. Now go back to the Apple TV app and you should see that the checkmark has been removed from the episode.

I usually start with the last episode and work my way back to the first but I don't think it makes a difference which order you do it in.

Yeah, my workaround requires that the app be partially integrated with the Apple TV app. You need to be able to start playing the episode from the Apple TV app.