Apple has released updated version iOS 6.1.1 for users. The iOS 6.1.1 specially designed for 4S devices, fixes an 3G issue while upgrade to previous version iOS 6.1 . Vodafone even suggest its 4S users not to upgrade iOS 6.1. This update contains file size is 23 MB.
The update is now available to download only for 4S users, not available for other iOS users.