Upgrading Tridion 2013 to 2013 sp1 is a fairly simple process. However, an issue exists in the installer and if you don’t take the following step, the upgrade installer will fail with a mysterious message.
This tip is thanks to Pankaj Guar
Rename the C:\Program Files (x86)\Tridion\config\Tridion.ContentManager.config to \Tridion.ContentManager.config2013GA
Run the upgrade script Tridion 2013 SP1\Content Manager\SDLTridion2013SP1CM.exe
Then after the upgrade merge any custom settings in the file, such as Event System config, GUI Extensions, or Razor Meditator.
This is much faster and better than uninstalling 2013 GA and installing 2013 sp1.