In the form builder, under the form fields, you'll find a "History" button. Click on it to view up to 25 events from your recent form activity such as adding or deleting fields or renaming the form.
Formstack allows you to undo field deletions up to seven days after the activity. You can double click on the deleted field within this 7 days and it will restore it back.
To restore a recent change there are two possible ways to make that happen. The first option is you can click on undo at the top of the page. This will undo the most recent change and continue to do so with each press.
The other option is you could go to the change you want to go back to and double click on that item in the list under activity and it will make the form as it was before that change.
Note: It is not possible to revert back a form or undo edits that are on previous activity category except deleted fields; only recent activities can be restored.