@MagnusAdmission do this, press the shirt icon, the one you press when you're changing clothes. Then press the DNA icon far left on your upper left corner. On your right you will see the name of the SIM press the surname, you will get a prompt with both the first and last name, press the last name, change the last name. Once you're satisfied with the name press the green check mark underneath. Press to get out as if you are done with remaking your sim. Agreed to the changes. Now you have the right surname.