first download the update
then dont install it but open your file manager and open the following path sdcard>andriod>data>com.nokia.update(or similar file)
there you will find a zip folder which you have to copy to your external sd card anywhere
then switch off your phone
then switch on the phone and as soon as you see nokia on screen press and hold the volume up key and your phone will go into recovery mode. after that choose the option update from external memory and browse your file and open it. now your phone will do the rest of the work updating itself
*use vol keys for mov up/down
powerkey for selection