1. Gestures: 2-finger tap to Undo, 3-finger tap to Redo
2. Top menu: Edit>Undo
3a. iPad: Undo button in right menu (long press to show Undo/Redo popover). You can also set showing Redo button in right toolbar in Artstudio>Preferences
3b. iPhone: Undo button in top menu (long press to show Undo/Redo popover).