If you want it to be install wide (this means all portals), you will have to upload it via FTP and overwrite the existing favicon.ico (which you need to discuss with your hosting provider if you don't know how to access it). If you want to do it per portal, you can place it in the Portals/[PortalID]/ folder as favicon.ico, where PortalID is your current portal you wish to replace. This could also be done via FTP. If you don't figure out FTP, you could add the ".ico" file type under Host Settings (permitted file extensions section) and then upload it to the root of the portal via the File Manager under the admin menu for that portal.