When you set up the quad in Vehicle set up wizard. Which aircraft type did you pick.
There are three ZMR250 set ups, I think the middle one is the correct one for a 3S setup.
The banks are changed by a three position switch on your radio.
When you went through the TX setup menu, it will have asked you to chose your flight mode switch.
This will be used to change your banks.
Banks are basically three different flight set ups, that you can switch between during flight.
You can have them set up for different reasons.
1) To test different PID setting, say you have your PIDs close but not perfect, you can run the close set up in bank 1, then use bank 2 and 3 to make some fine adjustments. If they don't seem to help, just flick back to bank 1 and continue to fly that pack with the close settings.
2) You can have them set up for different conditions. higher PIDs for calm conditions, and lower for strong winds.
For more in depth info, go to the Open Pilot Wiki page. All the info you would ever need is there
https://wiki.openpilot.org/display/WIKI/OpenPilot+Documentation