To exchange a scene trigger for a new one, open the scene you want to link with the new trigger. Hover over the trigger preview on the right hand panel and click 'change'.
(If you don't see the trigger preview or scene name in the right hand panel, you may have an object selected. Try clicking the background of the editor window to deselect it.)
You can then choose to upload a new trigger image for the scene, or use a generated one. Using a generated trigger is useful if you want to 'free up' the current trigger so you can link it to a new scene.
Please note it may take up to 2 minutes for your trigger to become active in your app, and you may need to restart your app to recognise the new trigger.