In many cases there's nothing frameworks need to do, it's just a matter of swapping out the HTTP server (e.x. https://github.com/expressjs/express/issues/2364#issuecommen...) or maybe even just sticking an HTTP/2-compatible reverse proxy close to the app servers.
Now, if you want to take advantage of HTTP/2 features like server push that's another story.
Now, if you want to take advantage of HTTP/2 features like server push that's another story.