Mac OS upgrades

Where can I get Mac OS upgrades for my MacBook Pro early 2015?

I got a new MacBook Pro and turned in my early 2015. So it had everything taken off of it. The trade-in didn't go through so now I have my early 2015 back and am trying to upgrade to higher versions of OS, which I had on it before. I can't find matching upgrades in App Store.

Pete