First, open the lesson from which you want to copy/move parts in the edit screen (editor).

On the left-hand side of the screen are the thumbnails of the lesson. Select the items you want to copy/move.

Good to know: the action described must be done within one tab of your browser. You cannot drag/move/copy between two tabs of your browser.

This also causes a yellow bar to appear at the top of the screen, including the Move and Copy buttons. Click one of the two buttons. You can also use the key combination CTRL+C to copy.

Now, via My LessonUp, go to the target lesson, the lesson in which the slides are to be placed. On the left, between the thumbnails, you can place the copied slides anywhere. You can also use the key combination CTRL+V to place them.