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.
Galaxy S2 remains one of the best selling devices till date. It really brought Samsung at the top. And while Samsung has forgotten this, some developers have not. They have continued with the developments of this device and have come up with the Omni ROM which will update this device to Android 5.1. Follow the guide below.