hi, i have been working on Xbee for quit a long, and i have also faced the problem you are facing, and i found the solution, here it goes
frist see that no other Xbee is connected to your laptop or is active.
second remove the port of the Xbee you want to change the frimware and replug it in.
then, open the recovery xbee and then choose the required function of the Xbee and click recover, it will shown a dialoque box at that instance click the reset button on the Xbee module, then that should work fine and the new frimware will be uploaded on to your xbee.
finally, remove and replug it in you will see that the friware has been changed..........