How do you Change the Administrator User Name and Password if you can not see it on the users list and do not know the previous one?
In short, you don't.
That's the entire point of having a secure computer.
The long answer is you have to wipe the machine & start over. Nothing on there belongs to you, not even the OS itself, & you would have to provide the old owner's credentials for any updates.
If you are no longer in touch with the previous owner, you'll have to start from step 6. Fortunately you can actually just start from there on an old Mac. On a new one, you would need to get the original owner to release it first.