That's done by hovering over the Admin Panel link (in upper right of site) to get the dropdown to show.
Then click the 'Main Settings' option.
On the Main Settings screen, under the line of orange buttons will be an option "Website Name:". Just put the name you want in the entry field to the right of that label, then scroll down and click the orange "Save" button.
To get back to your site, click the 'View Website" (white lettering) on the top line towards the right side.