How do I Change my last name on my account

See the following instructions from Apple for information on changing Apple Card information: Update your Apple Card account information - Apple Support

including the following from that support article:

If your name or phone number changes, contact an Apple Card Specialist at Goldman Sachs to update your account information.

Hello,

To update your name you will need to contact an Apple Card Specialist and will likely need to provide legal documentation related to your name changing. It will likely be helpful to have a copy ready to be sent in when you make the request with them.

Update your Apple Card account information - Apple Support

"Update your personal information

If your name or phone number changes, contact an Apple Card Specialist at Goldman Sachs to update your account information.

If you no longer use the email address that's associated with your Apple ID, you can also update your email address."

I hope this helps.