how do you change the template form on an existing form

I would like to change the template of an existing form. How do I do that? 

You can change the current theme of the form, but not the template. To change the theme, click on the Form Designer then go to Themes and select one theme.

The reason why you can't change the template of the form is, the form fields are directly connected to the submission data. If you remove or re-create some of the fields, you will lose the associated submission data of those fields from your Submissions page.

If you would like to change the form template, then you can clone one of the templates from the template gallery and use that form.