If often feels that the world works in contradictory ways and for me this belies the notion of creation by any deity. Why would it be so flawed if it were planned? So I guess if I had the power to change one law of nature it would have to be something which would be an undiluted force for good for the world at large. Something that would benefit everyone.
I do not know the technologies of what I want to suggest but would hope you get my gist. I think the focus must be on health and in particular of resetting the body defence so it could resist cancer cells. The destruction of cancer as a killer disease would make such a difference. The heartache, the unnecessary early deaths, the loss of people in their prime or before they have reached fulfilment would transform our society and give us hope to re-direct resources to solving other health problems.
The truth is though this is a dangerous power and could easily be used irresponsibly. Too many decisions are subject to the vagaries of politics rather than the essence of decisions themselves so it could be apocalyptic in the wrong hands should this power be enacted. Maybe we have to leave it to chance and enjoy whatever happens to us!