Do you already have a live Shopify Store, but want to make some big changes?
You can do this, without disrupting your current store by making a copy of the 'Live Theme' and work on the new unpublished copy, while your original Store is still working for you online.
You can make all the updates and changes you like and publish the copy when your ready, to over-ride the original Store.
Here's How To Make A Copy Of A Live Theme In Shopify
Open your store and click on 'Online Store'
Go to 'Actions' next to your theme and click 'Duplicate' from the drop down menu
Scroll down to see the copied theme click on 'Actions' of the copied theme and select 'Rename' from the drop down menu
Rename your copied theme
Now, you can click on the 'Customize' button next to your copied theme and make the changes you want, without interrupting the live site
When you are happy with your updates, exit the Customization and click on 'Actions' next to your copied theme.
Select 'Publish' from the drop down menu, this will over-ride the current live theme and make your copy Live