Working on your clients' live stores
If you want to work on a live store without affecting it, then you have two options:
Option 1: Work with a development store
Download the theme from the Themes page in the live shop, and then upload it to your existing development store. Now you can work on a copy of the live shop’s theme in a dev environment.
When you're ready to launch the updated theme, download the theme from your development store and then upload it to the live shop as the primary theme.
Option 2: Use a Theme Editor
If you're a Mac user, then you can edit your theme from your desktop using the Desktop Theme Editor for OS X.
You can also use Shopify Theme Kit to modify your theme with a local text editor of your choice. It then pushes your edits to your development store using our API.
Shopify supports multiple themes, so you can make a copy (duplicate) of your theme on the Themes page in your Shopify admin, and edit that unpublished theme.