So iOS5.1 launched last week and was available for immediate download. Many users have complained about an error when trying to use the SOFTWARE UPDATE feature on the iPhone for iOS5.1. This is a quick workaround that could resolve the problem as it did for me.

• Open Settings > Wifi
• Click on the little blue arrow on the right of the active connection name.
• Go to DHCP > DNS
  • My DNS setting was 10.0.0.2, but yours could be different.
• Change this to either the Google DNS setting which is “8.8.8.8″
• or use the open DNS setting which is “208.67.222.222″
• Now go back to Settings > General > Software Update and try again.

This resolved the issue with my iPhone 4, which is now completely up to date with iOS 5.1

Read also: How to download iOS5.0.1 and update your iPhone or iPad