Assuming you have an SSL certificate already downloaded and available on the server.

root@www:~#nano /etc/apache2/sites-available/default-ssl.conf

# line 3: change admin email

ServerAdminwebmaster@srv.world

# line 32,33: change to the certs gotten in section [1]

SSLCertificateFile /etc/letsencrypt/live/www.srv.world/cert.pem
SSLCertificateKeyFile /etc/letsencrypt/live/www.srv.world/privkey.pem

# line 42: uncomment and change to the chain-file gotten in section

SSLCertificateChainFile /etc/letsencrypt/live/www.srv.world/chain.pem

root@www:~#a2ensite default-ssl

Enabling site default-ssl

To activate the new configuration, you need to run:

systemctl reload apache2

root@www:~#a2enmod ssl

Considering dependency setenvif for ssl:
Module setenvif already enabled
Considering dependency mime for ssl:
Module mime already enabled
Considering dependency socache_shmcb for ssl:
Enabling module socache_shmcb.
Enabling module ssl.
See /usr/share/doc/apache2/README.Debian.gz on how to configure SSL and create self-signed certificates.

To activate the new configuration, you need to run:

systemctl restart apache2

root@www:~#systemctl restart apache2

http to https force redirect:

If you’d like to set HTTP connection to redirect to HTTPS (Always on SSL/TLS), configure each Virtualhost like follows. It’s also OK to set it in [.htaccess] not in httpd.conf.

root@www:~#vi /etc/apache2/sites-available/000-default.conf

<VirtualHost *:80>
RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [R=301,L]

root@www:~#a2enmod rewrite

Enabling module rewrite.

To activate the new configuration, you need to run:

systemctl restart apache2

root@www:~#systemctl restart apache2

Verify to access to the test page from a client computer with a Web browser via HTTPS. If you set Always On SSL/TLS, access with HTTP to verify the connection is redirected to HTTPS normally, too

Src: https://www.server-world.info/en/note?os=Debian_10&p=httpd&f=8