How to install lollipop on galaxy s2 gt-i9100

                

                                                      

Galaxy S2 (GT-I9100) users are in for some exciting news, as the industrious CyanogenMod developers have once again lived up to their fan expectations by unleashing the highly-desirable Android 5.1.1 Lollipop update via the unofficial CyanogenMod 12.1 (CM12.1) ROM.

The CM12.1 ROM gains precedence as Samsung has abandoned any hopes of reviving software support for the ageing Galaxy S2, with the last official update being the Android 4.1Jellybean

Never eless, discerning Galaxy S2 users can enjoy the experience of the latest Lollipop features on their handset by installing the Android 5.1.1 based CM12.1 ROM, using the step-by-step guide provided below.

The icing on the cake is that CM12.1 ROM can be used without the need to install Google Apps, which is the with other custom ROMs in them

Pre-requisites

• The instructions given in this guide are meant for the international variant of Galaxy S2 (GT-I9100) only. Verify device model number by navigating to Settings > About Phone
• Ensure the phone has at least 80% battery charge to prevent unexpected device shutdown (due to battery drain) while installing the ROM
• Back up data and settings using a recommended custom recovery tool, like ClockworkMod (CWM) or TWRP, as it creates a complete image of the existing phone setup. In Recovery, choose Backup and Restore and tap Backup
• Verify correct USB drivers for the phone are installed on the computer (download Galaxy S2 USB drivers)
• Enable USB Debugging mode via Settings > Developer Options (if running Android 4.0 or above) or Settings > Applications > Development (if running Android 2.3 or earlier)
• Ensure that the device is rooted with CWM recovery installed and its bootloader is unlocked

Downloads Required
Android 5.1.1 CyanogenMod 12.1 ROMGoogle AppsSteps to Install Android 5.1.1 based CyanogenMod 12.1 ROM on Galaxy S2 GT-I9100Step 1 - Download Android 5.1.1 basedCyanogenMod 12.1 ROM for Galaxy S2 on the computer.
Step 2 - Connect and mount thephone's USB mass storage on computer with the USB cable.
Step 3 - Now, copy the downloadedCM12.1 ROM zip file to the root folder on the phone's SD card.
Step 4 - Power off the phone and disconnect its USB cable from computer.
Step 5 - Now boot the device intoRecovery Mode by pressing Volume up, Home and Power buttons on your phone.
Step 6 - Once the phone enters into CWM Recovery, perform a full data wipe task by clicking Wipe Data/Factory Reset. Use the Volumekeys to navigate between options and the Power button to select an option in the Recovery Mode.
Step 7 - Then, perform Wipe Cache Partition and wait until the process completes.
Step 8 - Now perform Wipe Dalvik Cache as well. To do this, go to the'advanced' option and then click Wipe Dalvik Cache. This will help in preventing boot loop issues or any boot ROM related errors during firmware installation.
Step 9 - Then return to the main recovery menu and choose Install Zip from SD card.
Step 10 - Now click Choose Zip from SD card. Use volume keys to navigate to the CM12.1 ROM zip file that you copied earlier to your phone's SD card and choose it by tapping the Powerbutton. Confirm the ROM installation on the next screen and the installation process will begin.
Note: Repeat Step 9 and Step 10 toinstall Google Apps (Gapps) as well.
Step 11 - Once both the Gapps and ROM are installed, go to "+++++Go Back+++++" and select Reboot System Now in the Recovery menu.
The phone will now reboot and the first boot may take about five minutes to complete. So, please be patient.
Your Galaxy S2 GT-I9100 is now running the latest Android 5.1.1 Lollipop via CyanogenMod 12.1 ROM. Go to Settings > About phone to verify the firmware version installed.
[Source: XDA]