Maurizio,
In /etc/apache2/apache2.conf please check that you don't have this line twice (duplicated):
LoadModule harbour_module /usr/lib/apache2/modules/mod_harbour.so
You don't have to copy all the examples to /var/www/html. Simply go to /var/www/html and do this:
sudo ln -sf /home/$USER/mod_harbour/samples modharbour_samples
Now from your browser go to:
localhost/modharbour_samples
If you do the same operation (create a symlink) for mod_harbour.so and libharbour.so.3.2.0:
cd /usr/lib/apache2/modules
sudo ln -sf /home/$USER/mod_harbour/linux/Ubuntu 18.04/mod_harbour.so mod_harbour.so
cd /var/www/html
sudo ln -sf /home/$USER/mod_harbour/linux/Ubuntu 18.04/libharbour.so.3.2.0 libharbour.so.3.2.0
then to upgrade mod_harbour you will only have to do:
cd mod_harbour
git pull