How to update iOS from Software Update on your iPhone or iPad
Launch Settings from your Home screen.
*Note : Make sure you have made iCloud backup.
Select & tap on General.
Tap on Software Update.
Tap on Download and Install.
Enter your Passcode if required.
Tap Agree to the Terms and Conditions.
Tap Agree once more to confirm.
How to update iOS device using iTunes on your Mac or Windows PC
Launch iTunes on your Mac or Windows PC. (* Notes :Make sure you have the recent iTunes backup.)
Connect your iOS device (iPhone/iPad/iPod) using USB to Lightning or USB to 30-pin Dock cable.
Click on the device icon at the top left to enter the tap device.
Click on Check for Update.
Next, Click on Download and Update.
Click Agree to the Terms and Conditions.
Enter the Passcode on your iPhone or iPad if required.