If your site is currently using a theme and layout design and you have had a template applied in the background then these are the steps for previewing it using your current content.
1) Access the Site Design area
Select the Site Settings (cog) icon from the left hand navigation, and click on Site Design > Templates
N.B. If the Site Settings or Templates area is not available to you it could be that you don't have permissions, so please either talk to your site administrator or email email@example.com to ask for these to be enabled.
2) Select to show the available templates click on the 'Select/Edit button which will take you to the templates that are installed on your site. You will need to do this each time you access this area until a template is actually applied to your site, then this step will be unnecessary.
3) To preview the website homepage make sure the root group is selected and click on the preview (magnifying glass) icon next to the first template in the list.
4) To preview the content page template make sure you select one of the content groups and click on the preview icon next to the content template in the list.
Once you have the preview of one of the content pages showing you can then click on the menus to move around the content pages as you would if you were viewing from the front of the site, this will enable you to see all of the content pages as they would look with your existing content and the new template in place.
You may find you need to adjust images that are set to display at a fixed width as these won't be responsive within the new template. There is an article that will help you called Making Images Responsive.