Problem in Writing Xbee Pro Modem

I have Xbee Pro 2.4Ghz Modems. I wanted to establish a digimesh. Hence I planned to upgrade the firmware to XBP24 Digimesh (Firmware 8003). Earlier I was using 802.15.4 protocol(Firmware 10CD).
When i try to write XBP24-DM (8003) to the modem quite often it shows this message
“Unable to Communicate with COM Port…
Write Parameters…Failed”

Now I have burnt the firmware by trial and error method.
But I dont know why i was not able to write often.
So please let me know me if there are any regular procedures in upgrading the firmware.
Thank you.

I have encountered a problem with the series 1 that will not allow me to easily update the firmware. Although I have discovered that if you are using a development board, when you start the firmware upgrade one of the lights will light up for a few seconds. During that time press the reset button and that seemed to work for me.

The method you suggested works…