reformer
noun [ C ]
uk/rɪˈfɔːmər/ ussomeone who plans or supports changes to a system, law, organization, etc. in order to make it more modern or effective:
political/economic reformer He is known worldwide as an economic reformer who forged a landmark free-trade agreement with the United States.
The new governor had a reputation as a reformer who pledged to stamp out corruption.