A teammate cannot make it to the tournament? Want to swap out one of your players? Go to 'Your Tournaments' and then tap the tournament you want to edit the roster. Once you're on the tournament screen and see the timer and tournament details, tap on Edit Roster and swap out your teammates.
Important Note: You can swap players and edit your roster until 10 minutes before the tournament starts. Once the lock-out period has begun, the roster cannot be edited.