Change how my surname appears

You can edit last name in Mac address book , open address book select All iCloud type your first and last name in the spotlight search , the contact will open , click on edit , type your latest last name in the field , click on done , when you compose mail and send new mails to recipients the new edited last name will appear , the old mails that were sent will still have the same first and last name .

See the article Update contact information in Contacts on Mac - Apple Support

Create or edit contacts in Contacts on iCloud.com - Apple Support