On your iPhone or iPad, navigate to Settings > General > Software Update. Your iOS device will now check for updates.
Before beginning the update, be sure that your phone is plugged in/charging, you are connected to WiFi, AND you have at least 3.0 GB of free space.
Select "Download and Install" and then accept the Terms and Conditions to proceed with the installation. The download is rather large and may take several minutes to an hour depending on your internet connection. It took me about an hour to download. Again, you should perform the upgrade over WiFi to avoid ridiculous data charges.