1. If you already have custom _CustomHead.ascx file in your theme, use this one. If not, copy it from default theme to your custom theme. File location is Themes\{theme_name}\Views\Shared\_CustomHead.ascx relative to applications root folder
  2. Open file for editing and in the end of it add <link rel="shortcut icon" href="https://{application_url}/Themes/{theme_name}/Content/img/favicon.ico"/>
  3. Put desired favicon.ico in Themes\{theme_name}\Content\img\, again, relative to application root.

NOTE: Replace {theme_name} and {application_url} with actual values.