You can edit your teams as many times as you want, before the match deadline.
To edit your team:
1️⃣ Head to 'My Matches' and select the match under 'Upcoming Contests'
2️⃣ Click on 'My Teams' to the right-hand side
3️⃣ Tap the pencil icon on the team you wish to edit
4️⃣ Make the edits you wish to, save the changes and that's it. 😀