When working on an assessment, use the new History tab to switch between multiple submitted code states.
The History tab is found in the left-hand panel below the Description tab.
Clicking on the History tab brings up the Code History overlay.
The restore buttons (blue buttons with arrows inside) restore the code to one of the previously submitted states. If you have three submits, as shown in the screenshot below, you'll be able to restore to any of those three.
The more often you submit your code, the more options you will have for restore states. Submit early and often to take full advantage of this feature.
When you click the blue restore button next to any of the submitted states, the IDE will restore your code to that state. You'll get a popup letting you know the restore was successful.
You can also use the Diff buttons (document icon with a magnifying glass) to view a diff of the selected state versus what is currently in the editor.
Example: the right side of the diff shows the Current State. When you click the Diff button for the second attempt (middle row), this diff view appears.
The diff feature is available between the current state and any other submitted code state.