How To Create a Systemd Override File?

Procedure: 

You may override a systemd unit file using this command:

systemctl edit NAME.unit-type

For instance, to override a filesystem mountpoint named "mnt-volume123.mount" you will use this command: 

systemctl edit mnt-volume123.mount

Systemd automatically will create a drop-in file in /etc/systemd/system/<service>.d directory. Any statements made in the override file will take priority over the original vendor-supplied unit file usualy located in /usr/lib/systemd/<servcice>.unit-type. 

After this you need to reload all the daemons to read their modified unit files with this command: 

systemctl daemon-reload