Dawn theme - connecting form to the email address

I’m very new to Ghost (Pro) and my questions might sound to simple to others but I need some help.

I would like to connect my email address to the form. I know I need to update template custom-page-with-form.hbs

But I cant even get to the template to update it with my email address. It says I need to get an app to open it …

Please help if you can… or I will have to go back to Squarespace though I really don’t want to do it

Thank you very much