With the phone turned off,
Press and hold the camera button,
while holding that, press the power button to turn on.
This should bring you into bootloader mode (tri-color screen.)
It should say serial down at the bottom,
and you should plug it in to the usb cable to your computer.
the "serial" text should change to "usb"
Go to:
http://www.htc.com/us/support.aspx
and download the right rom for your carrier.
once that downloads, run that exe with the phone still plugged in
and saying usb at the bottom.
It should flash the phone just as if you had it hooked to activesync.
And stock roms have the radio's included and will overwrite/install a radio.
I never recommend updating just the radio only (like suggested above)
Those tend to cause problems.