To restart Apache on Ubuntu you have some options to execute.
A simple restart:
sudo service apache2 restart
You can also stop and start the service.
sudo service apache2 stop
sudo service apache2 start
If you just want to reload the configuration
sudo service apache2 reload
There is also the option to use systemctl
sudo systemctl restart apache2
And the same with stop and start for the service.
sudo systemctl stop apache2
sudo systemctl start apache2