Undo and redo

Undo backs out the last change to the sheet. The stack holds fifty steps, so you can reverse an experiment instead of rebuilding the page by hand.

Almost every change goes on that stack: inserting, moving, typing, deleting, formatting.

Where to click

Command	Keys	On screen
Undo	Ctrl+Z	Quick Access, to the right of Share; also Home tab, Edit group.
Redo	Ctrl+Shift+Z or Ctrl+Y	Beside Undo in both places.

On a Mac, use Command instead of Ctrl. The rest of the bindings are listed under keyboard shortcuts.

When the field wins

With the caret inside a math field or a text region, the first undo and redo belong to that editor and reverse the last characters you typed. Only once the field has nothing left to undo do further undos reach the worksheet: a moved region, a deleted block, a theme change. This is the usual reason undo looks broken. It is working, on a smaller scale than you meant.

Evaluate All adds nothing to the stack. It recomputes from the expressions as they stand. An unwanted answer is therefore undone by undoing the typing that caused it, not by unevaluating. Cut, copy, and delete act on the same selection as the Home Edit group, described under selection and clipboard. Both stacks are emptied when you open another worksheet, so finish reversing a mistake before you switch sheets.