kazulog@sv1:~$ sudo mkdir -p /etc/systemd/system/nginx.service.d
kazulog@sv1:~$ printf '[Service]\nRestart=on-failure\nRestartSec=5\n' | sudo tee /etc/systemd/system/nginx.service.d/override.conf
[Service]
Restart=on-failure
RestartSec=5
kazulog@sv1:~$ sudo systemctl daemon-reload
kazulog@sv1:~$ sudo systemctl restart nginx
kazulog@sv1:~$ systemctl cat nginx | tail -6
WantedBy=multi-user.target

# /etc/systemd/system/nginx.service.d/override.conf
[Service]
Restart=on-failure
RestartSec=5
kazulog@sv1:~$ systemctl show nginx -p Restart -p RestartUSec
Restart=on-failure
RestartUSec=5s