Apply custom template in my foler to an existing form

How can I apply a custom template we designed to an existing form in my folder. I tried and I don't see how to do that anywhere.

Can you help please. 

I would like to apologize, however, unlike Themes that can be applied to existing forms, Templates are to be used upon form creation. Please refer to the guide below for more information:

How-to-use-the-form-templates

If you have any further questions regarding this, please feel free to ask us anytime.

Thank you.