If you would like to unjoin a tournament you are fully registered to, you can do so prior to the tournament registration deadline and while not checked in.
Simply navigate to the tournament page, then hit the unjoin button as shown in the screenshot below. On mobile, this is found after tapping the Play button.
Hint: Navigating to a tournament you have joined is done quickly via the Joined Tournaments option found in the left sidebar.
If you are partially registered in a team-based tournament, you will find the 'Start Over' button in a similar place.
If you have checked in and wish to be removed, you need to contact the tournament organizer - they have the tools to remove you or your team.