You will need to download three primary files and save them to an external microSD card or your phone's internal storage: 1. Android 5.1.1 ROM Firmware

This is the standard method for installing any custom ROM like CM12.1 or Resurrection Remix.

How to Download and Update the GT-I9100 to Android 5.1.1 Lollipop Firmware

The original development section for the GT-I9100 on XDA is the holy grail. Look for threads with [ROM][5.1.1] in the title.

The ROM is checking your device model and fails. Fix: In TWRP, go to Mount and uncheck System . Or, update to the latest TWRP version for I9100.

Samsung stopped providing official updates for the Galaxy S2 GT-I9100 after Android 4.1 Jelly Bean. To experience Android Lollipop's features on this device, you must install a "custom ROM," a community-built firmware designed to bring newer Android versions to older hardware. Among the most popular for Android 5.1.1 were [7†L5]. This guide focuses on Android 5.1.1 as requested, but please note that this operating system version is also outdated, and many of the download links for these specific ROMs might no longer be active.

If you encounter bugs, try changing the custom ROM version or ensuring you have the latest modem/radio files. Battery Drain:

. Installing this on a different model will brick the phone.

You can update your classic to Android 5.1.1 Lollipop firmware by downloading and flashing a custom ROM like CyanogenMod 12.1 or Resurrection Remix . Because Samsung officially discontinued software support for the GT-I9100 at Android 4.1.2 Jelly Bean, an official stock firmware update for Android 5.1.1 does not exist. However, the active Android open-source development community on platforms like the XDA Forums has made it completely possible to run a highly stable, modernized 5.1.1 system on this legendary hardware.

Yes – with caveats.

Upgrading an older device requires precise preparation. Do not skip these prerequisites, as they prevent your device from getting bricked.

: Your Galaxy S2 must be rooted. If you haven't done this already, you'll need to root your phone using an appropriate method for your current version of Android.

Boot into (typically Power + Home + Volume Up). Wipe the system, data, cache, and Dalvik cache.

: Check your USB cable, try a different physical USB port on your PC, or reinstall the Samsung USB Drivers.