How can I update MacBook Air Lion when I can't review Apple ID with El Capitan?
You cannot 'inherit' anything on a new [used] Mac you bought.

You must 'nuke &amp; pave' - wipe it &amp; start over.
The seller ought to have done this before parting with it - because everything on there belongs to the seller, including the OS itself.

Full instructions at Apple KB - What to do before you sell or give away your Mac
If you have no contact with the original owner, you have to start from Step 6 [&amp; hope they had the sense to do the first 5].