CyanogenMod 13 official nightly featuring the latest Android Marshmallow version 6.0.1 is now available for Samsung's older Galaxy S2 handset bearing model number I9100, which is stuck on Android Jelly Bean, the last software upgrade after which Samsung stopped further Android support for this handset.
For the uninitiated CyanogenMod is based on Android Open Source Project (AOSP), with some additional features that are not available in the stock Android firmware, thanks to the developer in the Android community.
As for the new features, the CM 13 comes with Cyanogen Recovery by default, but you can go for alternative recovery mode, just in case you want, such as IsoRec. Root access is disabled by default in the latest CM build, but you can enable it if you want. For this go to Settings>> Developer Options>> Root access and set to Apps only or Apps and ADB. You might get an error message while flashing Gapps. This happens when your phone is running out of space in the system partition. To flash this ROM, you need at least 1GBG storage space for the ROM and Gapps.
- Copy the downloaded ROM and Gapps files to the SD card of your phone
- Disconnect the phone if it is connected to the computer
- Ensure that your phone is using a KitKat compatible recovery
- Boot the phone into recover mode
- Wipe all data before flashing
- Now install the CM ROM via recovery
- Flash Gapps from the SD card
- Finally reboot the phone
More details about the nightly build is available at XDA development thread.