how can i undo something i have deleted

If you mean that you deleted some part of your form and would like to redo this part, you can find the previous version of your form in the revision history. Please check this guide:

http://www.jotform.com/help/294-How-to-View-Form-Revision-History

If you deleted the form itself, you can find it in your trash. Please check this guide to learn how to reach deleted form:

http://www.jotform.com/help/134-How-to-restore-deleted-forms

If you mean something else from these, please explain in more details we glad to assist you better. Hope, it helps.

Thanks.