SSH in. ZNC daemonizes; you do not keep it in screen.
Start
znc
If you see ZNC is already running on this config, it is up. To bounce it, kill then start:
ps x
kill PID
znc
PID is the one on the znc line. kill -9 only if it ignores a normal kill. pkill znc works when you have only your own ZNC on the account.
pkill -SIGUSR1 znc saves config, it does not restart. See znc.conf.
After a password edit you must actually stop and start, not SIGUSR1.
Client still points at the same host:port — Connecting your IRC client to ZNC.