All sites now have an associated live development site for theme testing. For example, if your store is www.example.com, which checks out at example.crystalcommerce.com, then your development site is at example.dev.crystalcommerce.com.
The dev site theme must be stored in the dev/
folder in your theme directory. Just SFTP in and create the dev
folder and copy in your existing theme and start editing. You’ll need to copy in your theme.liquid, assets and any custom templates.