Algodoo

This is the first save file
Press 1 to delete the one corner.
Press 2 to delete the current athletes if you want to swap them out for others.
Press 3 to delete the intermost layer of the shapes
Press 4 to delete the middle layer in the shapes

If you want to change a marble, change the color, then go into Script Menu, and then replace whatever is in the "MaterialName" Section with the new colors name.

(I don't recommend deleting any layer unless a marble is stuck)