You can update the software either over the air or through computer. Updating through pc is the safest way.
To update using computer you will need the usb cable supplied with phone, a pc software(
Nokia Ovi Suite) from Nokia to back up contents and install the latest device software.
You can get the software and more comprehensive information here
http://europe.nokia.com/support/download-software/device-software-update?3110=
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