Remove 'Powered by Shopify' from your store in Seconds.
From your Shopify Admin, click Online Store Themes Actions Edit Code.
Open the Footer.liquid file.
Just before the {% schema %} tag you should see the following code line:
{{ powered_by_link | split: '" ' | join: '?ref=out-of-the-sandbox" ' | split: '"' | join: '?ref=out-of-the-sandbox"' }}