While it's much faster and easier to install iOS updates using the on-device Software Update feature if you backup using iTunes or simply have problems doing an over-the-air install, you can also update using iTunes.
How to update iOS using iTunes and a USB cable
- Back up your device to either iCloud or iTunes before performing any kind of update.
- Make sure you're on the latest version of iTunes. Go to About and then Check for Updates or visit Apple.com.
- Launch iTunes on your Mac or PC.
- Plug in your iPhone, iPod touch, or iPad.
- Once your computer recognizes it, click on the Summary tab if you aren't already there.
- Under the iPhone section, click on Check for Update.
- iTunes should recognize that there is an update available for, click on Download and Install on the window that pops up.
Once your iPhone, iPod touch, or iPad reboots, you'll be running the latest update!