The favicon, also known as a shortcut icon, is a 16x16 image that appears in a browser's address bar and when a user bookmarks your Appstore.
To customize your favicon, head over to your Appstore's Assets page, which you can through your Admin Settings > Interface tab in your admin dashboard.
Click 'New Asset' and you'll be able to select which file type you'd like to upload. Select 'favicon' from the drop down.
You can then either choose a file directly from your desktop or enter the URL of the image. Clicking update will immediately push your favicon live.
Remember, your favicon needs to be within the dimensions of 16x16, 32x32 or 64x64. Images of a different size will be warped to fit these sizes.