http://www.chess.com/tournaments/help.html#withdraw
withdraing from tournment help

How do I withdraw from a tournament?
You are allowed to withdraw from a tournament at any time using the Withdraw link located on the right hand side of any tournament homepage. However, withdrawing from a tournament while you have games in progress will automatically cause you to forfeit all of the tournament games on time which will negatively affect your timeout % ratio and your rating, even if no moves have been made.
if i withdraw from a tournment what happens to the games that are still going on?