On Plesk starting from version 12.5.30 update #28 you only need to go to your ssh console and enter this command to enable HTTP2 HTTP/2 or also called HTTP/2.0 on all sites that use SSL (HTTPS) and nginx.

To enable it type:
plesk bin http2_pref enable

To disable it again, simply type:
plesk bin http2_pref disable

If you don’t want to buy a SSL certificate, you can use Let’s Encrypt to get a free certificate for all of your pages, email, ftp and other services.