If you have an old Samsung Galaxy S2 lying about and you wish to give it a new lease of life then you might consider flashing it with Cyanogen Mod.
I did this recently but didn’t follow the instructions provided by Cyanogen because I couldn’t get the Heimdall suite working properly on a Windows machine.
Instead I decided to use Odin to follow the instructions and so I’ve set out my steps below to get Cyanogen 12.1 running on my Galaxy S2. Note that I take no responsibility to what happens to your device or data and you should make appropriate backups prior to this as the below instructions are destructive to the data on the phone.
First thing first, make sure you have installed Android Studio and the SDK Manager so that you have adb and the USB drivers installed. If you don’t want to install the entire SDK and Android Studio then you can download the platform tools directly from Google https://dl-ssl.google.com/android/repository/platform-tools_r23-windows.zip. The version linked has been verified to work with the Samsung S2 on a Windows 10 machine.
Next we need to get a hold of Odin, I’ve used Odin 3.10 to perform the installation. Getting a good copy of Odin is a high priority for me however I do trust the guys at chainfire and they package a version of Odin up with their auto root packages. Head over to https://autoroot.chainfire.eu/ and download a package with Odin 3.10 included. I chose to use the package for the S4 (i9505) (https://download.chainfire.eu/316/CF-Root/CF-Auto-Root/CF-Auto-Root-jflte-jfltexx-gti9505.zip).
Extract Odin from the autoroot package by extracting the zip file and deleting the .tar.md5 file that is included.
Next we need to flash a custom recovery.
Instructions found at https://wiki.cyanogenmod.org/w/Install_CM_for_i9100#Installing_a_custom_recovery suggest this recovery, and I had no problems with this image.
Once it is downloaded, open Odin as an administrator, put your Galaxy S2 into download mode (shut it down, hold the volume down + home + power buttons until a disclaimer appears, which you then need to accept). Connect the Galaxy S2 to the computer via USB cable. In Odin, you will see a message saying the device has been added. If not, then you will need to check your USB drivers (please Google for this problem).
In Odin, click on the AP button and then navigate to the recovery image you just downloaded and select it. Click start and wait for it to flash the recovery.
When Odin completes it will reset the phone, you will need to be ready to put the phone into recovery mode as soon as it is restarted. (To get into recovery mode hold volume up + home + power button until you see the Samsung logo, at which point you should release the power button and wait for it to enter recovery mode). If it doesn’t go into recovery mode and instead continues to boot then just pull the battery and retry entering recovery mode.)
If all has been successful then you should be in a custom recovery at the moment. Select the wipe/factory reset option and then select Install Zip option followed by Install Zip from Sideload.
At this point your phone should be ready to accept the Cyanogen package from your computer via adb. From the Cyanogen site (https://download.cyanogenmod.org/?device=i9100) download the Cyanogen 12.1 package (https://download.cyanogenmod.org/get/jenkins/135113/cm-12.1-20151116-SNAPSHOT-YOG7DAO1JN-i9100.zip).
Open up a command prompt where adb can be executed and then type the following:
adb sideload <path/to/cm-12.1-20151116-SNAPSHOT-YOG7DAO1JN-i9100.zip>
Once the installation is complete it will return to the parent menu and you can then choose to reboot the system.
If everything went to plan then your phone should boot into Cyanogen 12.1.
You may consider installing the Google Play Store by following this link so that you can installed Apps from the Play Store.