Can I change a thumbnail?

Setup page:

Click Select thumbnail button on the Project media card and you can select a new thumbnail from a selection, you can refresh the selection if you want more choice, using the refresh (arrow) icon

Edits page:

Click Select thumbnail button on the Edit media card and you can select a new thumbnail from a selection, you can refresh the selection if you want more choice, using the refresh (arrow) icon

Quick Edits page:

Unfortunately, you cannot currently change a thumbnail on a Quick Edit. We are currently building our new Editor which will include the ability to select a thumbnail. If you have any specific feedback on what else you'd like to see on the Editor you can provide the Product team with feedback on the feature.