The undo and redo buttons can be found in the bottom right of the editor canvas area (shown with the pink arrow in the screenshot below). Clicking the undo button reverts changes within the currently active component:
Things to note:
Undo History: Undo and redo persists the current editing session only - if you refresh the page, or open your letter in a new tab, the undo history starts from empty.
Undo within selected component: Select the component you want to revert before clicking undo. Undo/Redo is applied to the current selected component (indicated by the outermost a blue box).