MacBook Air setup

Using the Migration Assistant, I'm trying to transfer data from my old MacBook Air to a new one.

However, when I'm setting it up to run, I get an error message

"The selected source cannot be used for migration.

This source contains a newer release of macOS. Your Mac requires an upgrade before you can migrate from this source"

So my NEW MacBook Air has an OLD release of macOS? How can I upgrade my NEW MacBook Air to the NEW macOS?