If you are a business, need to use the theme on more blogs or remove the theme credit icon, swap to a Commercial licence
Go to your customize page, which is likely to be located here. Click the 'edit html' button beneath the theme tumbnail and replace all of that text with the code in the box above. Click 'update preview' and then 'save'. You may have to refresh the page to see all the appearance options and a functional preview.