You can revert a board draft to a previous published version of the board.

Step 1: Click on Published History

Step 2: On the right hand menu under "Published History" select a previous version of the published board.

Step 3: Click REVERT

Step 4: In the confirmation dialog click YES

Step 5: Another dialog will appear. Click YES if you want to return to authoring.

When you return to board authoring you'll see the reverted board.

Remember

The changes are not live until you click on PUBLISH CHANGES.