Apple today released iOS 6.1.1 Software Update which fixes an issue that could impact cellular performance and reliability for iPhone 4S.
System Requirements:
iPhone 4S
Update your device wirelessly (available in iOS 5 and later):
1. Ensure your device is connected to a power source. To avoid potential data costs connect your device to a Wi-Fi network.
2. Go to Settings > General > Software Update. iOS will automatically check for available updates.
3. If an update is available* you can tap Download to download the update.
4. After the download has completed tap Install to update your iOS.
Note: If you leave the update to download in the background, once the download has finished you will receive a notification saying an update is available for your device. Tapping “Details” will take you to “Settings > General > Software Update.” Tap “Install Now” to install the iOS update. If you decide to leave the installation for later “Settings” will display notification badge until the update has been installed.