If a checklist has yet to be submitted to eBird, you'll see it listed as 'not submitted' when you go to the "My Sightings" page on the mobile app. You can tap on the checklist to continue to edit it, submitting it to eBird when you're ready.
If the checklist has already been submitted to eBird, and you see the status as 'submitted' when looking at the 'My Sightings" page, you can still make changes to it, but you have to do it using the online tools on the main eBird website. Go to eBird and log in, and then go to My eBird and then 'Manage my checklists". Once there you can open the checklist you wish to edit and make changes to it just like you can on eBird.
We understand that this isn't the most straightforward way to edit eBird Mobile checklists; and we are planning to allow in-app checklist editing in future versions of the app. Stay tuned, and we'll do our best to improve the app as fast as we can!