There are two ways to undo a change if you've made a mistake or would like to go back to a previous version of your logo. Please note that these instructions only work on the logo editor page you are currently on, if you have navigated away from your logo after making a change, you will not be able to go back to the previous versions.
It is also important to keep in mind that making a copy of your logo using the make a copy button in the top left of the editor, will save a copy of your logo to you account, so that if you do make changes that can't be undone, you have a copy of your logo on your account that can be made available to you:
On the logo editor page, navigate to the right side of your screen.
Near the top of the page, you will see a curved arrow
Click this curved arrow to go back to your previous change
On the logo editor page, navigate to the menu to the left of your logo design
Click the History button to access your history of changes
You will see your previous changes appear in the History Variations menu, just to the right of the left navigation