Undo and redo

• Undo: Ctrl + Z (PC), Command + Z (Mac)
  
• Redo: Ctrl + Y (PC), Command + Y (Mac)
  

Hint: $\Uparrow$ = Shift, ←, $\rightarrow$, $\Uparrow$ and $\downarrow$ = arrow keys
Further keyboard shortcuts for this editor are available at ace: Default Keyboard Shortcuts.