You can always delete trains from your trip in the rail planner app. You can't book any reservations in the app, so no need to be afraid that you'd accidentally do that if you're playing around to try out the app.
Planning however, is best not done in the rail planner app, since it's often incomplete and generally not reliable. It's better to use national websites or apps; for international journeys, bahn.com or the DB Navigator app is a good choice.
To delete a journey: when viewing it click on the 3 dots in upper right corner and select delete journey.