Override a vendor systemd unit safely with a drop-in instead of editing the packaged file

Documented steps

1. Never edit files under /usr/lib/systemd/system directly — package upgrades overwrite them. Use a drop-in: `sudo systemctl edit myapp.service` opens an editor for /etc/systemd/system/myapp.service.d/override.conf and runs daemon-reload for you on save.
2. In the drop-in, include the section header and only the directives you change, e.g.: [Service] Environment=PORT=9090 MemoryMax=512M
3. To change ExecStart (or any list-valued directive like ExecStartPre) you must clear it first with an empty assignment, then set the new value: [Service] ExecStart= ExecStart=/usr/local/bin/myapp --flag
4. To replace the whole unit rather than layering on top, use `sudo systemctl edit --full myapp.service` — this copies the unit to /etc/systemd/system/myapp.service, which fully shadows the vendor file.
5. Verify what is in effect: `systemctl cat myapp.service` shows the base file plus every drop-in in apply order; `systemd-delta --type=extended` lists all overridden units system-wide.
6. If you created the drop-in by hand (e.g. writing /etc/systemd/system/myapp.service.d/10-limits.conf), run `sudo systemctl daemon-reload` yourself, then restart the service.
7. Revert cleanly with `sudo systemctl revert myapp.service` — it deletes drop-ins and the /etc shadow copy, restoring the vendor unit.