A favicon is a small, iconic, image that represents your website. Favicons are most often found in the address bar of your web browser. A well-designed favicon should be styled to match the logo or theme of your website. This way, visitors have a quick and easy way to recognize your website at a glance.
Gutensite recommends the favicon image be 48 x 48 pixels, it must be in the format .png, and be named "favicon".
Changing the favicon will require editing the design file.
Gutensite Control Panel --> Content --> Design --> Design Files --> Site --> Images --> favicon.png
After clicking on the file name's link, you'll be taken to a location where you can upload a new file. After the file has been uploaded, you'll be taken back to the Design Files.