http/aokp.co/devices/i9100
I assume you mean a ROM based on Jelly Bean, the one linked above is one of the most popular. It's called AOKP, I recommend you use the "Milestone MR1 build" for best stability. For instructions go here: http
/wccftech.com/install-aokp-jbmr1-build-6-android-422-jelly-bean-galaxy-s2-i9100/
Also bear in mind that there are now kitkat ROMs available if you want the latest from Google.
Incase you're not comfortable with custom stuff then for an official update you should connect your phone to your computer, download samsung "Kies" and it should prompt you to update it to the newest official, Jellybean 4.2 I believe.