download TFTPd32 from Google and install it on ur PC.. point the image that you have downloaded in the TFTP server..
connect a Ethernet cable between ur laptop and AP.. both should be in the same subnet.. and connect a console cable and get the hyperterminal console access and issue the command.. make sure you are able to ping the PC and the AP and vice versa!!
AP#debug lwapp console cli or debug capwap console cli
AP(confg)int fa 0
AP(confg-if)ip addr <addr> <mask> (same subnet as that of the laptop)
AP#archive download-sw /force-reload /overwrite tftp://<tftp ip address>/<image name.tar>