You never know. It happens automatically and you'll just notice a new firmware version in the Tesla app or when connecting to the gateway's IP address via your web browser.
I've been on 1.37.1 since May. I just saw the 1.38.0 release notes but never heard of anyone here posting about it. Looks like that was more for utility-grid connections for islands in the Caribbean. Maybe we'll bet 1.39.x soon. Maybe not.