1. Downgrade to 4.2.2 (10.3.1.A.2.67) by flashing the ftf file of your region. Search the thread to find ftf corresponding to your region.
for C6502 India
2. Root this firmware by cubeundcube method.
3. Take a TA partition backup using DevShaft's method. This is necessary for relocking your bootloader later.
4. Upgrade to Kitkat using PCC or SUS (or flash a Kitkat ftf which is meant for your region). Don't panic that you've lost root. Read further.
5. Make a ftf file of the update files that has been downloaded to your computer using flashtool (if you upgraded via PCC or SUS).
6. Unlock bootloader by visiting this link (take a backup of all your data before unlocking bootloader). (Unlocking bootloader voids warranty. But don't worry you can relock it if you have backup of TA)
7. Download the latest update super su.zip from Chainfire's website. Save it in external micro SD card of your phone.
8. Flash a custom kernel by DooMLoRD. This is a modified stock kernel with CWM recovery built-in. Enter into the CWM recovery and flash the super su.zip you downloaded earlier to root your phone and turn off the phone in recovery itself.
9. To get back to stock recovery, use the ftf file you downloaded (or created) at step 4. above and Flash only the kernel and fotakernel.
10. Now relock your bootloader by restoring the TA partition backup you made at step 3. Your phone is rooted now with locked bootloader.
>Users who already have a backup of TA, unlock your bootloader and follow steps from 7.
>Users whose bootloader can't be unlocked, follow first two steps, visit this thread to install [NUT]'s dual recovery.
and flash this pre-rooted zip