What to do if recently my employer told me that I have been getting overpai on my hourly rate sinc I started 9 months ago?

It depends by what they mean by "overpaid":

1) If that means that they think they were paying you too much for the job, but you were getting what they had agreed to pay you, they can't get the money back now. Example: they told you would get $15.00 per hour and that's what you received, but now they feel the job is worth only $10.00 per hour. In that case, since you were paid the then-agreed-upon amount, they cannot  retroactively change the agreement to recover money from you, though they can reduce your salary going forward. This situation includes if the supervisor/manager/etc. wanted you to be paid $10.00, but HR or payroll made an internal mistake and told you would receive $15.00 and that's what you got--that is, as long as you were receiving what you were told you would get, the money is yours.

2) However, if you were getting larger checks than you should have, and you knew or should have known you were being overpaid, they can recover the money from you. You are only entitled to the amount you agreed to work for, and an accounting mistake does not give you the right to more money. So say that you were hired for $10.00 an hour--you were told you'd be paid $10.00 per hour and knew that was your rate. However, they accidentally processed your payroll at $15.00 per hour. Since the agreement was that you'd work for $10.00 an hour, that's all you are entitled to, and the employer may get the extra $5.00/hour back from you.