How to update Samsung Galaxy SII SHW-250K from ICS to jelly bean?
You can simply flash win Odin GT-I9100 firmware.
Firmwares you can get from sammobile

Odin you can get from http://web.djodjo.org/?a=download:android:tools:win:bootloadertools

get Odin307

You can also check this tutorial for flashing with Odin.