How to update a SSL Certificate from a private key?

A client sent me a new SSL certificate for his domain site1.hisdomain.com.

The A-DNS to this subdomain points to my own server, running Ubuntu/Apache2.

My Apache .conf for this domain site1.hisdomain.ch is this:

UseCanonicalName Off

<VirtualHost *:443>
     DocumentRoot /var/www/site1/
     ServerName oursite.myserver.com
     ServerAlias site1.hisdomain.com
     <Directory /var/www/site1>
        Options +FollowSymlinks
        AllowOverride All
        Require all granted
     </Directory>

     ErrorLog ${APACHE_LOG_DIR}/site1-error.log
     CustomLog ${APACHE_LOG_DIR}/site1-access.log combined

  <FilesMatch ".php$">
         SetHandler "proxy:unix:/var/run/php/php7.4-fpm.sock|fcgi://localhost/"
      </FilesMatch>

SSLEngine on
SSLCertificateFile /etc/ssl/site1.hisdomain.com.pem
SSLCertificateKeyFile /etc/ssl/site1.hisdomain.com.key
SSLCertificateChainFile /etc/ssl/site1.hisdomain.com.csr

</VirtualHost>

He sent me a PFX file, which installed now a local certificate in my Keychain (a private key, RSA). How i can updated now the SSL certificate on my server from this file without crashing everything?

I overtook the server from a friend and have not really expirience how to update now the certificate.

Thanks!