Can anyone tell me how to change the operating channel(CH) in Modem configuration tab?

Actually I am using
Modem Type: XB24-B

for one module I am using
Function SET: ZIGBEE ROUTER/END DEVICE API
CH: 0
PAN ID: 0

and for another module
Function SET: ZIGBEE COORDINATOR API
CH: B
PAN ID: 0

How to change CH?

You set the CH by setting the SC value to the channels you want the coordinator to select from.