Yes, but only before the game starts. If you click the 'leave room' button (door and arrow symbol in the top left of the game page) you'll be given a confirmation message asking if you wish to leave. If you confirm your choice to leave, you'll be removed from the Tournament and will get your buy-in back.

However, if you lose connection or close the game window without selecting to leave the room, your Tournament will still be in play and you won't get your buy-in refunded. Instead, you will still be in the Tournament and will continue to receive cards and post blinds and antes. When the action is on you, you will be automatically folded if you are facing a bet. The system is designed this way so that you don't miss out if you lose connection - You can return to the site at any time and can re-take your seat at the Tournament as long as you still have chips. You will be able to go back in through the Poker Lobby and re-join your previous game.

As the games cannot be paused when you leave a room, you may find a reduced amount of Chips due to active Blinds.