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.