You can take back almost anything you do in the editor.
The buttons
Undo and Redo are at the top. They are greyed out when there is nothing to undo or redo.
The keyboard
Press Ctrl+Z (or Command+Z on a Mac) to undo, and add Shift to redo. This works when you're not typing inside a box or on the page's own text. In a text box, the browser's own undo does the job.
What can be undone
A change to the design, a font pack, a moved section, a hidden section, an added section, a removed section and an edited line of text, to name some. Dragging the dot in the photo focus window is one step.
After you remove something
When you remove a section or a block, a small notice at the bottom says what was removed and has an Undo button. It goes away by itself after a few seconds, and stays while your pointer or keyboard focus is on it. Press the x to dismiss it.
Each page has its own list
The home page and each added page have their own Undo list, so Undo only takes back changes on the page you're on. Switching pages doesn't undo anything.
Undo isn't the same as Discard
Discard changes throws away your whole unpublished draft and goes back to the live website. See How do I preview, undo or discard changes?.