How do I change my payment method on my 2018 federal amendment so I can file by mail? It was originally selected to pay by direct deposit, but I am now getting a refund?

An amended tax return, Form 1040X, can only be printed, signed, dated and mailed by you to the IRS, it cannot be e-filed. The IRS will take up to 4 months to process the amended tax return.

See this TurboTax support FAQ for amending a 2018 tax return originally completed and filed using TurboTax - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-a-return-in-turbotax/01/27579

Payment of federal taxes owed on an amended return can only be paid by check or money order included with the mailed amended tax return.

A federal tax refund on an amended tax return can only be sent as a check in the mail to the address on the amended tax return.

look at form 1040X and you'll see there is no  bank info line for tax due or for refunds.