Perform a Wipe Data/Factory Reset , Wipe Cache Partition , and go to "Advanced" to Wipe Dalvik Cache .
For a fully modern OS, or Android 11/12/13 custom ROMs exist for the GT-I9100, though they require repartitioning the internal storage due to the small system partition. gti9100 firmware 511 download upd
The CyanogenMod website is no longer active, and development for these ROMs has stopped. If the original links from the search results are broken, you can find a variety of newer and existing custom ROMs for the i9100 by searching on XDA Developers Forum [5†L9]. Perform a Wipe Data/Factory Reset , Wipe Cache
So, when you search for gti9100 firmware 511 download upd , you are looking for a custom Android 5.1.1 ROM update ZIP file for your Galaxy S II. If the original links from the search results
Android 5.1.1 offers significantly better security patches than Jelly Bean.