If you plug your iPhone into your computer and update with iTunes it doesn’t require very much free space on your phone because the update files are actually downloaded to the computer and the phone just accesses them to update. So, are you ready to update without deleting all 740,368 of your precious pictures or deleting half of your 283 apps? No problem! Just follow the steps detailed below.
Steps for how to update to iOS 8 using iTunes on your computer:
Back up your iPhone with either iCloud or via iTunes
Make sure you’re on the latest version of iTunes.
Launch iTunes on your computer.
Make sure your iPhone is plugged in.
Click on iPhone in the top navigation.
Click on the “Check for Update” button under the Summary tab.
It should show that an update is available. Then just click “Download and Install” in the popup menu and agree to any terms or conditions.
To view the Apple support page for updating your iPhone click here