Once you have entered a contest, you can still update your entry and lineup, as long as the first game in the contest hasn’t started yet. If the contest has already started, you will NOT be able to update your lineup anymore.
Tap on the contest that you want to update your entry for from the Results screen:
Tap on the “My Entries” tab, which will bring up a list of your entries for that particular contest. Then Tap on the “Update” link on the entry you would like to update:
You will then be taken to the same Create Lineup screen where you originally created your lineup. From here, you can add/remove players for your lineup: