I bought a second hand macbook air 11. How do I change the previous owner user name on the main hard disk to mine?
Giving away / Selling your Mac / Resetting the computer to the factory condition
if you have a relatively newer machine, this will erase everything and get your OS back to factory reset (which you should do for a used computer)
reboot holding down command option r
open disk utility and completely erase the HD and format as mac journaled
exit then choose install OSX
Retrieving data ...