Every time the builder saves your data a snapshot is automatically created using WordPress Revisions API.
To view all your revisions click Revisions (clock icon) in the builder toolbar.
Select any revision in the list to preview it. The canvas should automatically update containing the selected revision.
Click Apply to use the selected revision and continue editing from there.
If you don’t want to apply the selected revision simply click Discard or click anywhere on the canvas to return to your latest changes.