I updated one of my old FusionPBX from 4.2 to 4.4 (proxmox, lxc, debian 9)
mv /var/www/fusionpbx /var/www/fusionpbx-4.2
cd /var/www && git clone -b 4.4 https://github.com/fusionpbx/fusionpbx.git
chown -R www-data:www-data /var/www/fusionpbx
Then go to Advanced -> Upgrade and update the Source...