For any new dotnetnuke installation you have a default dotnetnuke logo look favicon showing on your web page's brower tab. To change it:
1. Create a new icon (any size) you would like to use;
2. Go to http://www.chami.com/html-kit/services/favicon/ and upload your new icon image and your new favicon will be created;
3. Download the newly created "favicon.ico", save it to your dnn root folder or any other sub folders if you prefer.
4. Open Default.aspx page, add the following line within<head></head>:
<link rel="SHORTCUT ICON" href="/Portals/_default/favicon.ico" />
(the path is where you saved the icon)
5. Upload the new favicon.ico; upload the updated Default.aspx.
Refresh your page and you should be able to see the change.