In the form editor, click the "Last edited [time]"  link at the top of your form to preview previous form revisions.

Select an option to preview your form in the Form editor. 

To revert to a previous version of your form, simply save the form once you've found the best revision.

A new revision is created every time you click the "Save" button on your form.

Form revisions are available on the Pro plan and above.