The second smartphone on the path to Samsung’s global smartphone domination is the Galaxy S2 – or properly, the Samsung Galaxy S II. Released in April 2011, this is a veritable old geezer in smartphone terms, but it is still alive and breathing, thanks to these new CM13 nightly builds that have been developed especially for the Galaxy S2, or lovingly called the I9100.
Nearly 5 years old now, the Galaxy S II’s last update was to Android 4.1.2 Jelly Bean. But XDA developer “Lysergic Acid” has been committed to bringing custom ROMs to the I9100, as he was also the one who developed CM12 for the same device. Through his work, it seems that your old Galaxy S II will still be able to enjoy Marshmallow flavors, which can't be said for other old devices out there. These are unofficial nightlies, of course. CynogenMod is probably the easiest custom ROM to tweak for older devices, being based on the Android Open Source Project and very nearly keeping all of the stock programming in there.
Flashing CM13 nightlies to your Galaxy S II will be no easy feat, and even the developer has some strict instructions. First up, your Galaxy S II will need to have, at the very least, a custom recovery compatible with KitKat builds (which means CWM or TWRP). The developer also asks for users to make sure their software before this is as close to stock as possible.
Check out the official XDA thread at the source link below. You can also find the download links there. Just make sure you follow the instructions to the hilt. Happy flashing!