You can unregister yourself for a tournament before deck list submission closes through your Player Dashboard. In your Player Dashboard, click on the tournament name to go to the tournament dashboard. Click on Your confirmation to view your registration confirmation. If deck list submission hasn't closed yet, click Unregister from this tournament.

If deck list submission has closed, you will need to reach out to the organizer and request that they delete your registration. Once that’s done, you should verify the registration is no longer in your Player Dashboard, and the League ID is removed from your profile.

One caveat – you cannot participate in another league's tournaments if you have already participated in three leagues' tournaments.