Changing/ editing/ removing the initial User name set at the beginning of setting up my macbook the first time out of the box.

How can I change the name that was set while setting up my macbook pro???

I had someone to pick up my macbook for me and for some reason he decided to open the box and turn on and set up my Macbook to check if it has any problems before taking it out of the shop. I changed the user name but even my phones Airdrop name was changed to that guys name. I changed one or two settings as well trying to fully remove the initial name, but I still see that name in Activity Monitor> User.

I have no idea how to fix it other than Factory reseting which is inconvinient.

Has anyone has this problem? I would appreciate your input.