SGS2 I9100 Guide: Updating to Latest Firmware

Samsung Galaxy S2 firmware can be updated by user by connecting the device to PC with KIES software installed.
You can download latest KIES from here.
My SGS2 was supplied with DXKG3 firmware, at present the latest official firmware for my device is DXKI2.
SGS2 white edition usually supplied with DXKI2 and therefore already on the latest firmware at present.
To upgrade the firmware, you need to have stable internet connection, since i need >3 times to successfully download the firmware.
What you need to do just fire up the KIES software, and it will automatically check the firmware updates, and if your device has new firmware, it will ask you for upgrade. 

The firmware itself is big, around 300MB. Just be patient.
As far as i know, the firmware upgrade is non destructible, which means won't wipe your device.

This is an official upgrade method, of course there's another "unofficial" way, which is using ODIN.
Me myself haven't tried the second method yet, i used ODIN just to flash CF-kernel for rooting the devices and CWM does the rest.