I e-filed my taxes yesterday. I put in my bank info for the payment to be made on April 18th. After I submitted my taxes, I realized that I underpaid. What should I do?

You can either cancel your direct debit, and then go on the IRS website to pay the full amount you owe.  Or, you can just pay the difference on the IRS website or by check with a 1040-V payment voucher.

If you want to cancel the direct debit from your original return and then go on the IRS website to pay.  To cancel your original direct debit:

Call IRS e-file Payment Services 24/7 at 1-888-353-4537 to inquire about or cancel your payment, but please wait 7 to 10 days after your return was accepted before calling. Cancellation requests must be received no later than 11:59 p.m. ET two business days prior to the scheduled payment date.

Then you can go on the IRS website to set up a new direct debit for the full amount of taxes you owe.   You can also pay the difference you owe on the IRS website.

https://www.irs.gov/payments

Or, if you want to mail in the difference owed, you can send in a check with the 1040-V payment voucher.

https://www.irs.gov/pub/irs-pdf/f1040v.pdf