Go to the tournaments page, there you will see all of your "Upcoming tournaments".
(these are the ones that haven't started yet).
Click on the tournament you want to withdraw from. This opens up a new window. On the right hand side there will be a large box with the words "Withdraw" in it. Click on that.
You can only safely withdraw from a tournament as long as it has not started yet or
if the tournament has started, as long as you have no current games, you can withdraw and not lose points.
If the tournament has started, and you do have current games, you will lose ratings on all games from that tournament.
I joined a tournament but it hasn't started yet
I'd like to leave it, how can I do that ?