Unfortunately, that's not how taxes work. You're only allowed to claim credits and deductions in the tax year they apply to, and income needs to be reported in the tax year it was received.
For example, if you made a sizeable charitable donation in 2019, but forgot to include it on last year's return, you can't claim it this year, because the donation occurred in 2019.
Likewise, you can't report income you received in 2019 on your 2020 return, nor vice-versa for that matter.
Either way, if you already filed the prior-year return for the tax year in question, you'll need to amend that return.