Currently default template is coming from page.php file and full width template is coming from page-templates/full-width.php
Now to change the default layout to full width template. Exchange the code of the above files starting from the line get_header(); to bottom of the page . After that, go to full-width.php file and find the below line in line 3 of that page :
Template Name: Full Width Page
and change it to :
Template Name: Page With Right Sidebar
