Assuming you uploaded the favicon, favicon.ico to /assets/images
the next thing is to link up the favicon in the layout page.
So within the head tag, you will have the following code:
<%= favicon_link_tag image_path("favicon.ico") %>
The above code snippet will output:
<link href="/assets/favicon.ico" rel="shortcut icon" type="image/vnd.microsoft.icon" />
Hope this helps