If you are using Samsung Galaxy S2 and looking for Marshmallow ROM than here is a good news for you people. CynogenMod make CM13 nightlies which is based on Android 6.0 Marshmallow.
It is good to see the effort from CynogenMod for making this ROM because, today the life cycle of a smartphone is around one year. After each year every company updates their flagship devices with newer ones. Reminding you Samsung Galaxy S2 is five year old phone launched in 2011 with Gingerbread.
In 2011 Galaxy S2 was launched with dual-core 1.2GHz processor and 4.3″ 480×800 display you can hardly be found in the lowest of the low-end Android phones. It does have 1GB of RAM and 16/32GB of storage though which make the device compatible for the modern Android ROMs like this one. you can get the ROM by clicking on the source link below.
Samsung Galaxy S2 Gets Marshmallow via CM13 nightlies ROM