You can edit your lineup at any point before the start of the contest by clicking on the contest in the Upcoming tab.
Once the contest has started, you can edit players from your lineup, by going to the Live tab. You will only be able to swap players in and out whose match has not passed the scheduled kick-off time.
As an example:
Let’s say you select Wayne Rooney from the 12.30pm fixture, and Daniel Sturridge from a 3pm kickoff. Rooney plays as expected, but as the Man Utd game finishes, you learn that Sturridge has been sidelined by a strained pinky toe. Now you can swap out Sturridge and replace him with somebody else within budget from any eligible fixture that hasn’t yet passed scheduled kick-off time.
In the event of delayed games you cannot change a player for another with the same scheduled kick-off time, even if one of the kick-off times is subsequently changed. However if the game is postponed until the next day, late swap will become available ahead of the newly scheduled game.