lease buy out that later trade in

2013 leased a car.  2016 bought out the lease.  2021 traded the car for another car.  Cars used partially for self-employment business. 2017 was less than 50%. I've used only the standard mileage deduction.  How to I account for the trade in on the 2021 tax return.  (Perhaps I should have done something different in 2016? I did not report the buyout.)