This step-by-step guide will help you to update Galaxy S2 with the Model number GT-I9100 to Android 6.0.1 Marshmallow using CyanogenMod 13 ROM.
Samsung Galaxy S2 is now quite old to get an official firmware update from Samsung but thanks to Android developers community who are still working on Galaxy S2 developments and releasing new Custom ROM updates based on the latest version of Android OS, this time, Marshmallow update has been rolled out.
To install the update make sure you’ve already installed the latest version of TWRP recovery to flash the Marshmallow ROM.

Galaxy S2 GT-i9100
Getting Started:
Step 1: First of, connect your phone to PC and copy the CM13 and Gapps zip packages to your phone.
Step 2: Reboot Galaxy S2 into Custom recovery. Power off your phone and hold the Volume Up+Home+Power buttons simultaneously for a few seconds until the recovery options appears on the screen.
Step 3: It is highly recommended that before making any changes you should take Nandroid Backup of your Current ROM.
Step 4: Clear the system cache and factory reset it in order to prepare your phone for new installations.
Step 5: Finally your phone is ready for update.
Step 6: When CyanogenMod 13 ROM is successfully installed, then use the same method, as in Step 5, and flash the GApps zip file.
That’s it! You’ve now successfully updated Galaxy S2 to Android Marshmallow. Who would have thought that you would be able to install the latest version of Android on such old device, that’s the beauty of Android. Taste Marshmallow features right on your Galaxy S2, don’t forget to share your experience with us in the comment section below.
[source: xda-developers]
Hello, I'm Imran Aftab, a tech enthusiast using Android, iOS, and Windows. Hardware expert for Gaming & Crypto mining rigs. I have been writing on tech since 2013, starting with ohguideme, then Androidcentral. I have written and published several guides and tutorials on how to root Android, flash custom ROM, recovery, and jailbreak iPhone, and have written several guides on how to bypass FRP. I also worked in a phone repair shop, so I have pretty good experience with mobile software and troubleshooting. So, all the guides you see here have been tested and confirmed to work.