try google

http://www.google.ca/search?hl=en&site=&source=hp&q=how+i+can+upgrade+samsung+galaxy+s2+gt-i900+to+Jelly+Bean&oq=how+i+can+upgrade+samsung+galaxy+s2+gt-i900+to+Jelly+Bean&gs_l=hp.3...2247.2247.0.3145.1.1.0.0.0.0.124.124.0j1.1.0.les%3B..0.0...1c..6.hp.blSjps9IYBY