How can i update my OS (gingerbread)
You probably have to use ODIN. I've done it myself, and it worked for me. I used it to root my phone (SGS2). I've postponed it for months, afraid it would brick my phone, but it didn't. 


Make a backup of all important data. 
Find a good manual on Youtube, XDA-developers or via Google. Compare them, and see what they say. 
Be prepared that it can go wrong. If it goes wrong, it could mean that your phone is bricked. 


I have a friend who bricked his Samsung phone, and Samsung repaired it without making problems over a rooted phone. 

Google won't update your phone. Samsung has to prepare an update because of phone specific drivers and their own specific apps. That's the only official way to update, and if Samsung doesn't make that update, there is no alternative but to use a ROM.