Do you have iTunes installed in your computer?
If you don't have, you can download and install iTunes by going to
Once you have downloaded and installed iTunes, open iTunes in your computer and connect the iPhone to the computer. Your iPhone will now be listed in iTunes. Click on the iPhone logo listed in iTunes, and to the middle of the screen, you will find an "update iPhone" option. Click on it to start the update.
Let me know if you find any difficulty.