/quote detach .. - Detach a channel from ZNC
/quote detach ##graphics
Pay attention to the slashes as they matter.
Connecting back is then simply
/join .. (or connecting through Textual GUI click).
/znc jump - Reconnect to the network. Useful for activating changes you made to the ZNC itself. Such as changing real name or nick.
/msg *sasl set NICK PASSWORD where NICK is IRC nick and PASSWORD is IRC password to make ZNC authenticate with SASL where you are identified before being fully connected to the network.