Just copied and pasted from somewhere for people who want to upgrade immediately.
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You can try:
How to change CSC on samsung Galaxy s2 (GT-19100) (India)
1) Just type *#272*Device IMEI number# at phone dial pad. (to get imei numbr, type *#[member='066709']# on your mobile)
2) Select CSC where ics is avilable ( I selected NPL)
3) Click Install
4) Phone will be rebooted by itself. (The phone will be formatted by it, so dont forget to backup your data)
5) Once the phone has restarted, just connect it to pc via kies, and it should show you or ask you to update.
6) Thats all, when you update to ics, again change the csc code to INU thats India.
(Not sure if it works on gs2 GT-19100G, but sure it works on GT-19100. .)