Nginx added support for HTTP/2 in version 1.9.5 and superseded the SPDY (pronounced SPeeDY) module, meaning that, as of 1.9.5, SPDY is no longer available and we now have to use HTTP/2.
If you installed Nginx via a package manager this module is most likely enabled; if you compiled it yourself please make sure you compiled Nginx using the --with_http_v2_module configure flag.
Similarly to SPDY, HTTP/2 requires the use of SSL, which is good practice regardless. These days, SSL certificates can be issued for free by services such as Let's Encrypt so this is highly recommended.
To enable HTTP/v add the http2 flag to the listen directive:
listen 443 ssl http2;