I'd replace the author of the Gospel of Matthew. I would create a new version of Christianity with different values:
- Liberalism, including support for pre- and extra-marital sex, divorce, homosexuality, freedom of speech, and justice reform
- Free market capitalism, with genuinely free choices.
- Globalism and free trade. Disdain for national borders.
- Empiricism and valuing knowledge. Some basic scientific facts to get things started, including some verifiable using 2nd century technology but also basic medicine, electricity, and evolution.
- The equal value of all human beings, explicitly condemning sexism, ableism, and racism in all their forms as well as emphasising that wealth is often due to fortune rather than value. Emphasising the need to include the whole community in decision making.
- Acknowledgement and explanation that gender, race, and disability are social constructs.
- Secularism and tolerance of differing religious beliefs; actively encouraging free thought.
- Robust defence of your values.
- And of course, retaining Christianity's existing values for love, compassion, and charity.
That's probably the single thing you could do to best improve the world. Obviously ideally you'd repeat the process with other major religions but Christianity is by far the most popular and so the most capable of doing good.
Perhaps a more honest answer is that I'd give myself perfect instructions for investing in Bitcoin, and then betting on Leicester to win the 2015/16 Premier League title. With that money you could basically do anything. I think I'd hire all my friends and create a not-for-profit news website that spanned the whole world (although tailored for an English-speaking and particularly British audience).