I already e-filed, but I made a mistake with payment method (wrong checking account #). How can I correct this before posting date of 4/18 ?

[CapB]

Once your return is accepted, you can't change any of the payments details, but, you can cancel the electronic withdrawal payment and use another payment method.

You can call IRS e-file Payment Services anytime at 1-888-353-4537, 7 to 10 days after your return was accepted to cancel your payment. 

You must cancel your payment 2 business days prior to the scheduled payment date.

Check out "Cancellations, Errors and Questions" from IRS link below "Pay Taxes by Electronic Funds Withdrawal"

https://www.irs.gov/uac/pay-taxes-by-electronic-funds-withdrawal#errors

To setup new payments directly through IRS and state websites:

Visit www.irs.gov/Payments and choose a payment option instead of paying through TurboTax. 

Guidelines vary per state. Check out your state's link below for more information on changing a direct debit payment date:

https://ttlc.intuit.com/replies/3302452