Decrease amount of CO
2 in the atmosphere by 1%.
That'll take less than a year to get the stuff down to pre-industrial levels, and can be used every once in a while as needed after you get it down until we've weaned ourselves off fossil fuels. (Reduce the deviation of current climate behavior from the pre-industrial standard by 1% for a while to reverse any points of no return we may have crossed, too. You probably can't eliminate the effects of global warming entirely, but you would be able to turn it into nothing more than the odd freak heat wave, which would be mostly harmless.)
It sure
sounds like going after various social metrics is the way to go, but there might be some nasty side-effects because of how quickly things would change. For example, if you used your power to decrease the Gini index of the United States, it would take you about a month and a half to get down to the level of Sweden or Denmark. Doing so would also entail one of the world's biggest and fastest wealth redistribution programs, and probably crash a lot of financial markets. You might end up worse off than before.
I'm having trouble thinking of a bad side-effect of increasing HDI, however. Sub-Saharan Africa still hasn't hit 0.500, and even just bringing it up to the level of Brazil (about 0.75) would do a massive amount of good in less than two months. Bringing the entire world (0.702) up to Canada level would take less than a month. (As a side note, looking at
this infographic gives you a good idea of just how far not just the developing but also the developed world has come in the past three and a half decades. Brazil in 2015 has a higher HDI than the UK did in 1980.
(Nobody's hit an HDI of greater than 1 yet, but it's not actually impossible! The benchmarks for an HDI of 1 are a life expectancy of 85 years, a GDP per capita of $75,000, and 18 years of schooling on average. I wouldn't be surprised if we see a country hit 1.00 within the next half century. With our magical power, we could hit that benchmark within two months.)
Increasing the IQ of the human race could be useful. I would worry about the economic fallout, since transforming the average human's lifestyle from that of a factory worker making $3000 in Mumbai to a PhD student living in the Bay Area is almost certainly going to have some side effects. (Of course, we could just incrementally increase the stability of the world economy, which would get rid of any nasty recessions within a month or two, assuming you could measure it.)
The whole point is this: six months with this power and you've solved pretty much all of humanity's biggest problems. After you're finished with that, keep yourself young forever and increase the value of your trust fund by 1% a day. (Such a trust fund would have an annual return on investment of 3678%. Might want to shortcut your path to obscene wealth by working for Wall Street.) After that, you're kind of stuck with doing things for shits and giggles.