I don't really agree with resurrecting shows for a number of reasons, but if I had the power I'd change the history of two shows in particular.
First, I'd go back to 2005 and tell the Arrested Development writers that they've got one more season of 18 episodes. It would get rid of season 4 (which I really appreciated even if it wasn't on top form) and season 5 (of which I've only seen the first half), but that's a small price to pay if the result is three wonderful seasons of one of the defining comedies of the 21st century so far. What we currently have are two perfect seasons (1 & 2), two decent but messy seasons affected by creative problems (3 & 4), and a watchable but pretty drab fifth season.
Then I'd go back to 1996 and tell Bill Oakley and Josh Weinstein that they could have as much money as they wanted for three more seasons of The Simpsons, so long as they kept 'Behind the Laughter' as the final episode of both season 11. Then I'd cancel it, make sure the show went out on top, and didn't gradually become a completely different show by the turn of the millennium. Or, even without giving the original showrunners three more seasons, just grab Mike Scully after season 11 finishes and tell him to produce no more.