I amended and efiled my amended return but want to change the payment date. Why does it want to efile again? Can you not just change the date to submit payment?

No, once you send the file the IRS will take over.  It's like dropping it in the mail box.  Once it's in you cannot access it.

Once the tax return has been filed the payment date cannot be changed.

Go to this IRS website for how to cancel the payment - https://www.irs.gov/payments/pay-taxes-by-electronic-funds-withdrawal

• Once your return is accepted, information pertaining to your payment, such as account information, payment date, or amount, cannot be changed. If changes are needed, the only option is to cancel the payment and choose another payment method.
• Call IRS e-file Payment Services 24/7 at 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.

After the payment has been cancelled, go to this IRS website to make the tax payment - https://www.irs.gov/payments