How to re-send an old submission to email?

You can resend the email by editing the submission, it is not necessary to make any change, simply going to the submissions page, editing it and clicking on the submit button will trigger the email again. 

Here's a guide that will help you with editing submissions: https://www.jotform.com/help/325-How-to-Edit-Submissions-Made-on-Your-Forms 

Also, make sure to have enabled the option to send the emails when the submission is edited, this is checked on the emails wizard: 

Hope this helps.