Owner / User system name change

I want to change the owner / system name on my new MacAir. Long story made short. Merged files from a HP Netbook that was a 'display' model and that machine had been named 'mj', now my Mac reflects that as it's name. I've gone to Users & Groups and changed the name of the computer, but 'mj' STILL appears on file directories.

I am new to the wonderful world of Mac and need help. Lots of help .....

There's a difference between the system name and the user name. Which one are you looking to change (since you seem to mention both)?

Read this to learn how to change the user name. The machine name can be changed by going to System Preferences > Sharing.

The system name.

Done!!! Thank you very much!