Galaxy S2 was launched in 2011. It is powered by a 1 GHz dual core processor and 1 GB RAM. It had 16/32 GB internal storage options which could be further expanded by 32 GB via microSD card. It also had a 8 MP rear camera and a 2 MP front facing camera. It originally ran on Android 2.3.4 and was later upgradable to Android 4.1.
Samsung stopped further upgrades to this device. However the latest version of Android (5.0.2) can be installed on the Galaxy S2 via the CM 12 custom ROM. The nightlies are out for this device which each nightly solving some of the problems associated. Follow the guide below to install CM 12 on the Galaxy S2.