Once you get your serial cable, connect it to the router and to your pc.  Load up hyperterminal (or a similar program) and set it to the com port of your serial port.  Then boot up the router, itll tell you when to press a key so you can get in and reupload the new firmware via xmodem.  Your going to have to look up the specific commands like 'at go' and 'atur1'.