It is quite easy to undo changes in Divi as long as you have not exited the Divi builder page.
Click on the clock icon in Divi’s bottom panel.
This opens a side panel showing Editing History.
It lists the changes you have made to the page. Click through the changes and you can see a real-time preview of how the page will look if you were to press the green tick button. You can also press the undo and redo buttons at the bottom to cycle through the changes. Once you are satisfied, click the green tick button to apply the undo. To exit without making any changes click the red close button.