After five years on an F1 student visa (and 183 days present in the preceding three years), you can file as a Resident, so your 2019 and 2020 tax returns can be completed in TurboTax as a Resident.
However, you are correct that probably in 2016/2017 you should not have filed a Resident return. The IRS can generally audit you for the last three years, and may have done so if your return was substantially incorrect. You probably don't need to amend those returns.
Click this link for more info on IRS Audits
This link has more info on F-1 Visa Holders
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