This depends on whether the contest you are participating in is a late swap contest or not. If you see an open padlock next to the slate name, that means it allows late swap!
In late swap contests you can edit your roster and add or remove players up until their individual game starts. For example, if the contest starts at 8pm, but your player doesn’t start their game until 9pm, then you can simply swap them for another player at any time before 9pm.
If, however, the contest is not a late swap one, all lineups will be locked once the contest starts and no further edit to your roster will be possible.