You can check your firmware info by typing *#1234# , if you'll get like this example firmware details below, you can upgrade to I9100UHKK3.
PDA: I9100UHKE1Latest firmware: I9100UHKK3/I9100CHEKL1/I9100UHKK1/I9100UHKK3
Firmware build: Gingerbread.UHKK3
Country/Provider: Chile - Entel PCS
Product Code: GT-I9100LKLCHE
Version: Android 2.3.5 Gingerbread
Build/Released: December 2011
or visit sammobile.com to get the latest updates
You need Odin downloader tool to manually flash this ROM on your device, just be aware that this is a bit risky for ordinary users and serious issues might happen like resulting to brick the Galaxy S II Android phone if not enough knowledge on how to use it. This is ROM is for advance user only for they already got an experience on how to tweak the phone's firmware.