The question is different from the one I had, but IMHO, I think you should pick something significant yet not to negative (unless you have a really strong "comeback" story). I would recommend against picking something mundane such as "getting better grades" or "finding a better date during college" etc...
The key is to state the problem/issue you wanted to change, talk about what actions you took to change it, and end with some kind of "learning" that made you a better leader/person/manager/etc...
For example, I may pick "taking a semester to go abroad during undergrad" as something I wish I had done. (rephrase it to something you would change). Since the essay is 250 words, you can at most spend 2-3 sentences on the intro and conclusion. Save the rest for the "how you would change things".
So State the problem: If I could have changed anything, I would have gone to Country X for a semester abroad, because Reasons A, B, and C
How would you do it? I would take advantage of Program J at my school, go to Country X, studied their culture through classes such as W, U, and V. And so forth...
Conclusion: If I studied abroad, I would have gained skills D, E and F early on, but even though I didn't, I learned those skills through my job/extracurricular/volunteer work in the past N years and that's why I realized I should have done the study abroad back in college. (end on a positive note that shows you still gained the benefits of whatever you would change, but just wish you did it earlier or something like that).
This is very rough, but hopefully it helped in kick starting this tricky essay.
ALWAYS REMEMBER: Talk about your reasons for doing things, explain your thought process, and show what you learned/gained from the process and how it makes you a better leader/person.