"If you could start your career over again, what would you do differently?"
This depends on several factors. If your university major is different than your career choice, then you could answer that you would choose a different major to prepare you for your choice in career. Or if you started at a small company and didn't have many opportunities, you can say that you would have started at a larger company. This is not a significant question so a regular answer should suffice.
Short Answers
"I started my career at a small company. I feel I would have benefited more if I started at a larger company. A large company usually provides training that I didn't receive, so I would like to have started my career in a larger company."
"I didn't know how much I liked working with computers until I finished college. If I had known earlier, I would have liked to have worked with computers earlier in my career."
Long Answer
"Well, I majored in chemistry and now I'm working in the marketing department for a telecommunications company. I had a lot to learn in the beginning, but I quickly learned what I needed to know to be an effective member of the marketing team. However, I always felt that if I majored in business or marketing, I would have started off with a better foundation. I'm great at what I do, but if I could start over again, I would probably choose to have majored in business administration."
This is an acceptable answer because the answer doesn't display any problems in the work history. Many people change careers or go into a field that is unrelated to their major. This is common, so answering this question using this answer is common. If you don't want to use this common answer, then thinking about a scenario early in your work career that you would have done differently might be more effective.