To list installed software type:
dpkg --list
dpkg --list | less
dpkg --list | grep apache2

To delete the software named apache2, use the apt command or apt-get command as follows:
sudo apt-get remove {package-name}
sudo apt-get remove apache2

You may see info on screen that read as follows:

The following packages were automatically installed and are no longer required:
apache2-bin apache2-data apache2-utils libapr1 libaprutil1 libaprutil1-dbd-sqlite3 libaprutil1-ldap
Use 'sudo apt autoremove' to remove them.
Hence, run the following command to remove package that no longer needed as I deleted the apache2 and free up disk space:
sudo apt-get autoremove
df