Yavor Ivanov

Hello, I am looking for an advice of how I can claim a refund for all my NI contributions. Some details: I lived (studied at university and worked) in the UK from 2005 until 2016. During this period I'd worked part-time as a student and full-time after graduation. I moved back to Bulgaria in 2016. As I don't plan to ever come back to the UK to live there, how can I claim a refund for all my NI contributions? Many thanks!