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I'm not sure what you meant by you have clicked delete on the preview window. Are you referring on accidentally deleting the form? If you do, you can restore it by following this guide: http://www.jotform.com/help/134-How-to-Restore-Deleted-Forms
If you are referring to the actual changes on the form, if there's a previous version of your form that had those changes that were successfully saved before, you should be able to revert back to that state by following this guide: http://www.jotform.com/help/294-How-to-View-Form-Revision-History
Do let us know if that works.