What should I do if my professor changes the question mid-exam?
This does happen sometimes, despite a professor's best efforts to check the exam beforehand.  Professors are humans and make mistakes.

You can write a polite email to the professor (or whoever is in charge of grading the exam, if different), letting them know that you feel this had a disproportionate negative impact on your score.

That's about all you can do.  It is ultimately up to the professor (or grading committee, etc) to decide what to do about this issue, if anything.  They might:


Do nothing, reasoning that although the correction was unfortunate, it affected all students equally.
Give credit to students who made an appropriate attempt to solve the impossible version of the problem.
Adjust the "curve" or other statistical correction of the exam score to take this into account.
Discard the question's score, and reweight the scores on the other questions.
Discard the entire exam and hold a new one.
Discard the entire exam and reweight other exams in the course to compensate.


In principle, if you don't agree with the professor's decision, you may be able to appeal to some higher authority.  This would depend on your university's regulations, and my guess is that it would be unlikely to succeed, if the professor did anything halfway reasonable. I'd consider that any of the above options would satisfy that.