The following 2 buttons perform undoing/redoing editing steps.

The button Undo, which is enabled when at least one modification has been made, will roll the editing status back one step.
The button Redo, which is enabled when the Undo button has been clicked at least once, will re-apply all editing steps that were previously undone.

If the mouse cursor hovers over these buttons, a tool-tip will be shown containing a brief description of the steps that are possible to undo or redo.

Use the standard CTRL+Z key combination for undo and CTRL+Y for redo.