How do I input tuition expenses from prior years as a qualified expense against a 529 distribution I received this year?

[Carl]

Sorry, but it doesn't work that way. 529 funds must be used to pay qualified education expenses, in the same tax year the funds are disbursed.  So 529 funds withdrawn in 2017 would result in the beneficiary recipient of those funds receiving a 1099-Q for 2017, showing the amount withdrawn in 2017. The taxability of those withdrawn funds are then offset by the qualified expenses they were used to pay for in the same tax year the funds were withdrawn.

So any 529 funds withdrawn in 2018 will result in the beneficiary recipient of those funds receiving a 1099-Q next year. They will offset the taxability of the funds withdrawn in 2018, by claiming they were used to pay for qualified education expenses in 2018.