We should thank to Lysergic Acid, the XDA senior member for its efforts delivering the Android 5.1.1 Lollipop based on CM12.1 Nightly Build. He didn't make much changes on it, therefore you can expect the same experience ROM like the official CM12.1. Albeit it's still in beta stage, but if you find nothing too wrong with it then you can of course use it as your daily driver. However about its performance, do not expect a complete stable performance with it. Perhaps in the future update, the developer of this ROM will make it more stable and full with features.
And today, I will try to guide you to update your Samsung Galaxy S II GT I9100 to Android 5.1.1 Lollipop using this unofficial 12.1 ROM. The steps are compiled here and shared for anyone owning that smartphone.
Your phone should already been rooted and have installed a custom ROM, this post will use CWM recovery, but you can use other custom recoveries too like TWRP or Philz recovery, the process is the same.
Remember that a custom ROM is not developed and produced by Samsung, therefore everything offered with it is the result of modifications of the third-party Android developers with intentions to deliver their own version of Android software and or providing the Android version to the unsupported devices. Risks can happen for installing this ROM, neither I nor the developer of this ROM should be held responsible for any damages or bricks during / after the installations. You are the one responsible for the unwanted result if that really happen. So proceed at your own risk guys.
- You should install this unofficial CM12.1 ROM on Samsung Galaxy S2 GT-I9100, do not install it on any other models of the phone!
- Backup your existing data, like SMS, MMS, Contacts, etc before proceeding.
- Enable your Android USB Debugging mode first.
- Your device should already have a bootloader unlocked.
- The ROM developer didn't include Gapps on his ROM, thus we need to install it separately, the download files of Gapps will be found in the instructions below.
- Prepare your Windows computer, a USB cable and have installed a USB driver.
Files you need.
- Now connect your Samsung Galaxy S2 to computer using your USB cable data.
- When they are connected, now transfer the CM12.1 and Gaspp files to your device's internal storage.
- When all the files are transferred you can disconnect your phone from PC, then turn off your phone, now boot it into the recovery mode. To do use just press these buttons; Volume UP + Home + Power.
- Firstly perform backup by hitting the Backup and Restore option.
- When the backup process is done. You can now perform a factory reset by selecting this option; Wipe Data / Factory Reset.
- When done with cleaning, you can now hit the install zip from sdcard, to then choose the CM12.1 ROM ZIP you transferred earlier form computer.
- Repeat the process above to install the Gapps. Close your action with hitting the Reboot System Now.