Before You Update Your iPhone, Update iTunes
Make sure you're running the latest version of iTunes.
Apple delivers its iPhone software updates through iTunes, so the first thing you should do is make sure you have the most recent version of iTunes running on your computer.
To update iTunes, go to the "Help" menu, and select "Check for updates."
If iTunes says you have the most recent version, you're all set to move on to Step Two. If iTunes tells you that a more recent version of the application is available, download it.
Accept all prompts necessary to install the updated software. Note: Apple's updater is likely to suggest additional software that you can download (such as the Safari browser); none of this is necessary. You can download it if you like, but you don't need it to update iTunes.
Once the iTunes update has downloaded, it will begin installing itself automatically. When the installation is complete, you may need to restart your computer in order to run the new version of iTunes.