Your Pick'Em Pool selections are locked in on a game-by-game basis. Once a game begins, any picks you have on it will be locked in. You can edit any of your other picks on games that haven't begun, however.
For example, if you're in an NFL pool and make a pick on a Thursday night game, you'll still be able to edit your picks for Sunday after the Thursday night game is over. If you have a play on a Monday night game, you can still edit it after the Sunday slate is completed.