If you mean the Registered Owner, go to Start, Run, type regedit and navigate to :
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\WindowsNT\CurrentVersion and in the right pane, right-click RegisteredOwner and Modify the value. You can also change the Registered Organization.
Create a system restore point before making the change.
If you mean (in addition to yours) you have the previous owner's profile on the PC, just go to Control Panel, Users and delete it.