Ah, what would I do if I had a wish...
I would probably start by wishing that I had been on duty at the Chernobyl Nuclear Power Plant on the 26th of April, 1986...
I would then, using my knowledge of the RBMK style reactors, and the knowledge of the outcome of this unsafe experiment, prevent it from ever taking place...
This would then lead to the USSR not going bankrupt, because they wouldn't need to immediately pour billions and billions of Rubles into containing the disaster...
The USSR would (hopefully) still exist in our modern world, albeit heavily reformed...
The US couldn't run about doing whatever it wants all willy nilly...
Pripyat would still be a lush and burgeoning city, and the world would not have an irrational fear of Nuclear Power...
This would, in effect, let the development of safer and more reliable Nuclear Power continue as it should have, slowly but surely making dirty power (coal, oil, natural gas ect..) obsolete...
But we can only dream...