Edit → Undo (Ctrl+Z) or the toolbar button Undo to reverse a maximum of 100 editing actions to return to the preceding state. Complex operations like "Replace All", "Indent selection", etc are treated as a single undo event.

Edit → Redo (Ctrl+Y for Windows, Ctrl+Shift+Z for Mac OSX and Linux) to recreate a maximum of 100 editing actions that were undone by the Undo function.