Blockchain has been increasingly used as a software component to enable decentralisation in software architecture for a variety of applications. Blockchain governance has received considerable attention to ensure the safe and appropriate use and evolution of blockchain, especially after the Ethereum DAO attack in 2016. To understand the state-of-the-art of blockchain governance and provide an actionable guidance for academia and practitioners, in this paper, we conduct a systematic literature review, identifying 34 primary studies. Our study comprehensively investigates blockchain governance via 5W1H questions. The study results reveal several major findings: 1) the adaptation and upgrade of blockchain are the primary purposes of blockchain governance, while both software quality attributes and human value attributes need to be increasingly considered; 2) blockchain governance mainly relies on the project team, node operators, and users of a blockchain platform; and 3) existing governance solutions can be classified into process mechanisms and product mechanisms, which mainly focus on the operation phase over the blockchain platform layer.