Your mobile tickets will remain on the app for 30 days past the last date of travel.
Pulling down on the screen will refresh the app, download any new tickets and remove used ones.
You can also delete tickets from the app at any time by swiping left on a ticket and selecting Delete. Your tickets will be immediately removed from the app.
Once deleted you will not be able to download the tickets again, so make sure you don't need them before deleting.