Since you have not yet filed, simply subtract the amount the school refunded you in 2019 from the tuition paid in box 1 of the 2018 1098-T.
To do this in TurboTax, when you enter the 1098-T, click the "What if this is not what I paid" link in blue below the entry for Box 1.
Scroll down on the screen that opens and enter the lesser amount (see screenshot)
According to the IRS:
"Refunds received after 2018 but before your income tax return is filed. If anyone receives a refund after 2018 of qualified education expenses paid on behalf of a student in 2018 and the refund is paid before you file an income tax return for 2018, the amount of qualified education expenses for 2018 is reduced by the amount of the refund."
Since you have not yet filed, simply subtract the amount the school refunded you in 2019 from the tuition paid in box 1 of the 2018 1098-T.
To do this in TurboTax, when you enter the 1098-T, click the "What if this is not what I paid" link in blue below the entry for Box 1.
Scroll down on the screen that opens and enter the lesser amount (see screenshot)
According to the IRS:
"Refunds received after 2018 but before your income tax return is filed. If anyone receives a refund after 2018 of qualified education expenses paid on behalf of a student in 2018 and the refund is paid before you file an income tax return for 2018, the amount of qualified education expenses for 2018 is reduced by the amount of the refund."