Undo: Ctrl + Z (Windows) / Command + Z (Mac)
Step backward through multiple undos: Ctrl + Alt + Z (Windows) / Command + Option + Z (Mac)
Step forward redo: Ctrl + Shift + Z (Windows) / Command + Shift + Z (Mac)
Edit > Undo
Edit > Step Backward
Edit > Step Forward
History panel: click a previous state to revert
History panel: create a new snapshot to preserve a state
Edit > Keyboard Shortcuts to customize undo and redo shortcuts