If you need a board reviewed during a tournament, you can call the Director using the TD call function. Click the blue box with three white horizontal bars (the "hamburger menu") on the playing screen and select Call Director.
After the tournament ends, there's a 20-minute correction period during which Directors can still review boards and make adjustments if warranted. Once that window closes, the results are final and cannot be changed.
To reach a Director after your table has finished and you're back in the lobby, you can send them a private chat, by clicking their username in the chat area, or find them through the People tab.