Go to the HAProxy frontend settings. For each individual frontend (not just the primary frontend), scroll down or search for alpn using the on-page search. You should see:
Paste or append this content there:
alpn h2,http/1.1
It should now look this this:
Now Save the settings and reload HAProxy.
After you reload the pages for which you just activated HTTP/2 using Ctrl+F5, you should have a HTTP2 connection.