If you received forms 1098-T from your college for 2013, 2014 and 2015 and did not claim the American Opportunity Credit, you need to amend your tax returns for those 3 years. Please note that for 2013, you need to file your amended return by April 15, 2017 otherwise it will be too late. For 2014 and 2015, you have more time.
To amend your returns, please follow the instructions in this TurboTax FAQ:
https://ttlc.intuit.com/replies/3288565
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