First of all make sure that the following 3 requirement is fullfilled:

• A registered domain name with working HTTPS (TLS/SSL). HTTP/2 only works alongside HTTPS because most browsers, including Firefox and Chrome, don’t support HTTP/2 in cleartext (non-TLS) mode.

• Also you need at least Apache version 2.4.24

• If you run PHP in Apache via mod_php, you need to switch to FPM. That is not a bad thing. FPM is newer and faster.

Now lets do the configuratoin for HTTP/2

Switching Apache’s PHP Module from MPM Prefork to Event
Run the following commands:

sudo apt-get install php7.2-fpm
sudo a2enmod proxy_fcgi
sudo a2enconf php7.2-fpm
sudo a2dismod php7.2
sudo a2dismod mpm_prefork
sudo a2enmod mpm_event
sudo service apache2 restart

Installing and Enabling HTTP/2 in Apache

Enable the module mod_http2:

sudo a2enmod http2
sudo service apache2 restart

Enable the HTTP/2 protocol by adding the following to /etc/apache2/apache2.conf:

Protocols h2 http/1.1

Verify that HTTP/2 is Working

Cloudflare put together a comprehensive list of ways you can check a website for HTTP/2 support. The easiest to use are probably Chrome Dev Tools (network view, add the Protocol column) or the online test from KeyCDN.