how do i change and pay my taxes from bank account instead of mailing a check in

Yes, if you have already sent your return to the IRS and it has been accepted, it is impossible to make a change.

if you said you'd mail a check then you can just pay electronically from a bank account via EFTPS instead (eftps.gov).

You can also do estimated tax payments thru that platform also, no checks or vouchers required.  Just be sure if using EFTPS to make sure you apply whatever payment against the correct tax year.

That's ok.   If you said you will mail a check you can pay the IRS directly online at
https://www.irs.gov/payments