On the screen where you enter your Form 1098-T, click the link underneath Box 2 that says "What if this is not what I paid to this school?" It will then give you a box to enter the amount you actually paid.
But it's a little late for that now. The IRS wants proof of how much you paid. (They probably would have asked for proof even if you had entered the actual amount in TurboTax.) You will have to reply to the IRS notice with a copy of a statement from the school showing your payments, and maybe canceled checks or other proof of payment.