This example shows how to modify an existing service. Service modification are stored within `/etc/systemd/system`, in a single file or in a subdirectory named after the service. For example, this procedure modifies the `httpd` service.
This creates an override file `/etc/systemd/system/httpd.service.d/override.conf` and opens it in your text editor. Anything you put into this file will be *added* to the existing service file.
To completely replace (instead of just add to/modify) an existing service file, use `systemctl edit --full`, e.g. `systemctl edit --full httpd.service`. This will create `/etc/systemctl/system/httpd.service`, which will be used instead of the existing service file.