You can change the theme to a different one through the Administrator>Theme menu option (see the quickstart guide).
You can't edit and upload an existing template. If you need changes to a template then please email us as firstname.lastname@example.org.
In some instances, we may be able to make some minor changes for free. In other cases, bespoke work is charged for. Our price for a completely bespoke designed template is shown on our pricing page. Bespoke changes to existing templates will be charged as a proportion of this charge.