Install Nokia Suite (If you haven't already) from here:
http://www.nokia.com/gb-en/support/product/nokia-suite/Once installed connect your phone to your PC using your cable and using Nokia Suite connect within the program.
Once connected click the green button on the toolbar for software updates, it will check if there is a newer version for you phone and if so prompt you to install it, if not it will say you already have the latest version, click here to reinstall.
Click on "here" and let it download & install.
Don;t forget to backup your data first (this can also be done with Nokia suite).
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