In the Storyboard Edit View, it's not currently possible to undo an action. All changes - for example, adding notes, uploading images, moving/deleting frames - are saved in real-time and can't be reverted.

Storyboard Edit View 

In the Image Editor, however, you can undo actions! From the Toolbar, click the Undo icon or use the keyboard shortcut Command + Z. 

Image Editor 

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