I assume you have root access to modify configuration files. SSH to your server using root and go to the folder where vhosts of nginx are present. (on my server, it's /etc/nginx/conf.d/vhosts).
Open the .conf files and make the following change.
listen a.b.c.d:443 ssl http2;
So basically, you need to append ssl http2 to the line with listen under server.