Since you've already filed 2017, but I don't know where you are in the process, I've attached instructions for the entire process.
After you finish this step, you'll go to this section to change your AOC to Lifetime:
1. Click Federal on the left-hand side menu.
2. Click Deductions & Credits across the top.
3. Scroll down to Education and click Show more.
4. Click Start or Revisit by Expenses and Scholarships (Form 1098-T).The difference between the two credits will be reported on your 1040X.