If new iPhone software is available, you should have already clicked "Update."
You'll see a pop-up message from iTunes, notifying you that it is about to update your iPhone's software and that it will verify the update with Apple.
Click "Update" again to continue.
iTunes may then present you with information about the new features in the software update and the hardware required to install it. Make sure that you have compatible hardware before you continue. If you do, click the prompts to move forward.