How to change Xbee firmware without RS232 port?

I need to change the firmware of a Xbee3 to support 802.15.4. I have both:

But It seems that the usb board doesn't support changing firmware (since it doesn't have a DTR signal), and I can't find a computer with a RS232 D9 port (it seems they don't make them anymore). Is there any solution to changing the firmware of Xbee modules?

Thanks!

endrew:
But It seems that the usb board doesn't support changing firmware (since it doesn't have a DTR signal)

Buy a USB-TTL adapter that has a DTR signal.

It appears that XBees are 3.3V.
So, I would buy one of these Unbranded Semiconductors & Electronic Components for sale | eBay
The CP2102 is 3.3V on the RX and TX pins.