As long as the validity hasn't started yet (the start day is still in the future), you can cancel the activation. Go to My Pass and then tap on the 3 dots at the top right. There should be an option to cancel the activation.
If you want to give the pass back, I don't know how that works; there should be information though on the DiscoverEU website, because then they can give it to the next person on the waiting list.
