How do I go back and change my return to add my refund to next years return

You can make this selection in Other Tax Situations under Additional Tax Payments. 

If you have already successfully filed the return this cannot be changed. 

Or if you get the refund just send in an estimated payment.

https://www.irs.gov/pub/irs-pdf/f1040es.pdf 

Or you can pay directly on the IRS website https://www.irs.gov/payments

Be sure to pick the right kind of payment and year.....2020 Estimate