This can be an editorial, or creative change, but it needs to be somewhat plausible. No bringing people back to life or putting people on the books before their time

Seigel & Shuster Era

This one is easy. I would change the despicable treatment of the creators of Superman by National Comics to ensure they not only got a fair share of the profits of their creation, but remained the main guiding force behind the character. I have no idea whether that would have ended poorly for the character and would have resulted in Superman no longer being as large a figure in popular culture, but I would like to see what Superman would have become if Siegel & Shuster had been allowed to introduce Kryptonite (which they called K-Metal), and have Superman reveal his secret identity to Lois as early as 1940.

The Weisinger Era

While I think this was a particularly crazy, creative and weird period of Superman's history that produced some amazing classic elements that continue to this day, I have only heard horrible things about Mort Weisinger as a human being, so I would rather Julie Shwartz be given the reigns earlier to see what he might have accomplished with Superman with his stable of creators.

The Shwartz Era

Likewise, I would have liked to see what could have happened if Shwartz had left the books earlier, and handed the editorial reigns to one of his younger, hungrier proteges like Elliot S. Maggin, Len Wein or Marv Wolfman.

The Byrne Era

Creatively, I think John Byrne did wonderful work with Superman and brought a lot of new readers to the character that might otherwise have never been interested. However, I wouldn't have asked Byrne to reboot the entire mythology as DC editorial did. Byrne has been quite open about the fact that he would have been perfectly willing to take over Superman as he was, making changes to the status quo within his stories, much as was done with Batman, whose broad strokes history remained the same, but with specific retcons being made along the way to modernize the character. I think Byrne work on the character in retrospect often feels too much like a reaction against what the Pre-Crisis character was, rather than simply trying to tell the best Superman stories he could.

The Triangle Era

I think this era was largely successful, both creatively and commercially, but too much time was spent trying to make the character "cool" and "hip", which only had the result of making him look even less cool (I am looking at you Super-Mullet and 90s pony-tail). There was also too much focus on bringing back Pre-Crisis elements without fully bringing them back, which satisfied no one. Pick a lane. Lack of a singular creative vision here is extremely evident. It seems like neither DC editorial, nor the creative teams were on the same page, particularly once Loeb came on board.

The Post-Infinite Crisis Era

There was a clear direction at the start of this era. Johns & Busiek both seemed to be in agreement about what they wanted to accomplish, which was to stop the half-measures and equivocating and simply merge the Post-Crisis and Pre-Crisis elements of the Superman mythos into one cohesive whole, but they tripped right out of the starting gate when the Kuberts' family issues stalled the first story arc, and the new origin of Superman wasn't established until years later by Secret Origins, leading to pointless confusion. I would have Johns & Frank do the origin first, then allow the books to proceed from there. Without a strong foundation, its difficult for any revamp to succeed.