Hi, Anix. Check out the IOS Reference:
http://www.cisco.com/en/US/products/hw/routers/ps272/products_technical_reference_chapter09186a008022e0e3.html#wp2647917
The command for configuring fallback on a 1240 is copied below:
station-role
Use the station-role configuration interface command to set the role of the radio interface. Use the no form of the command to reset the parameter to the default value.
1200 and 1240AG Series Access Points
station-role
{non-root [bridge [wireless-clients] | wireless clients] |
repeater |
root [access-point [fallback {shutdown | repeater}] | ap-only] |
root [bridge [wireless-clients]] |
scanner |
workgroup-bridge}
I hope this helps!