i copied this out of the Big Lama manual. it is pretty self explanatory and simple to do. make sure that you "unbind" the Rx from the Tx first, just in case... then follow the re-binding instructions.
Disconnecting the binding
Switch off the transmitter.
Switch on the model.
Press the binding button for about 1 to 2 seconds with a
pointed object at the receiver component. The status
LED at the receiver flashes. This means that the
connection has been completely severed.
Re-establishing the binding
Disconnect the receiver from the power supply.
Connect the receiver to the power supply again. The status display flashes.
Switch on the transmitter.
When both lamps are lit permanently, binding has been performed and the control signals transmitted are received.
The remote control is now ready for use.