Update the software from your mobile phone, go to 1a.
Update the software using iTunes, go to 1b.

Tap Settings.

Tap General.

Tap Software Update.
If a new software version is available, it's displayed.
Follow the instructions on the display to update the phone's software.

Go to www.itunes.com on your computer.
Find the download section on the website to download iTunes.
Download and install iTunes on your computer.
Start the program iTunes on your computer.

Connect the data cable to the socket at the bottom of your mobile phone and to the computer's USB port.
Depending on your settings in iTunes, the program may suggest to update your mobile phone automatically.

To start the update manually:
Select your mobile phone in the menu to the left.

Select Check for Update.
If a new software version is available, it's displayed.
Follow the instructions on the screen to update the software on your mobile phone.