Instructions for amending are here:
https://ttlc.intuit.com/community/amending/help/how-to-amend-change-or-correct-a-return-you-already-...
Once you bring up your originally filed 2020 tax return in the amendment version of 2020 TurboTax, revisit the HSA section. I'm assuming that you had reported the $1,250 as a personal contribution for 2020, so remove that since that was actually a 2019 contribution.
