To change the name of the site, open the site in your Impress.ly account.
The name of the site can be seen at the top of the screen.
Just click on the name of the site, select the text, and type in the new name.
The name of your site has now been changed.
Keep in mind that the name of a site and the title of the site as seen on the home screen of your site can be changed independently of one another. To change the title of the site as seen on the home screen, please follow the instruction available here.