My there are two options and it will help you ..i am certain sure coz i have upgraded it
so my suggestions
* If your cell phone is in warranty period than you should go to care and they will update it for you or its not than try these solutions-:
(i) make backup of ur fones data in pc /laptop
(ii) Root ur device with super user or su.
(iii) Download SDK of the version 4.2 or any from XDA Dev teams website. there you will get further info .
2- (i) Download ANDROID MAGNET software if you wont be able to find it than
Go to Youtube and type ANDROID MAGNET software its video tutorial with site name it will help you...