The Samsung Galaxy S II GT-I9100 never received an official Android 5.1.1 update; its official support ended with Android 4.1.2 Jelly Bean. To run Android 5.1.1, you must install a custom ROM, which requires your device to be rooted and have a custom recovery like TWRP or CWM installed. Top Custom ROMs for Android 5.1.1 Several stable versions were developed by the community:
Flash the Firmware: Return to the main recovery menu, click Install Zip, find your downloaded ROM .zip file, and confirm the installation. gti9100 firmware 511 download upd
If you want a stable, updated system with the look of 4.1.2, consider: The Samsung Galaxy S II GT-I9100 never received
Experience: Provides a clean, lightweight, and bloatware-free experience that mimics stock Google Android UI. Alternative: Custom ROMs That Look Like Stock “511”