How to change the main user name of a used MacBook on macOS 10.13?
You can do the short name shuffle. I prefer this over trying to rename an existing account. Much better to make it “new” with the old files intact. 


Make a new admin - to rename the old account(s)
Log out of the old and in to the new
Use the new to delete the old account - answer “don’t change” the home folder when asked 
Open /Users and rename the deleted folder to be the new account short name of your choice
Create a new account - specify the new short name from above step


The system sees the user folder is present and asks you to confirm to reuse it, answer yes. Log out of the new admin and test the old documents with the new renamed account. No need to delete the temp account, but you can now if you please.