How to enter bank account to get 1040-X processed

Got an IRS paper letter saying they need my bank account info to process my 1040-X refund, but I can’t find any clear place on IRS.gov to add or update direct deposit details.
Has anyone dealt with this before?
Do I need to call, mail something back, or is there a specific IRS portal/form for amended return bank info?
Trying to avoid delays or accidentally doing the wrong thing.