Load by Serial rs232 the fwf file

Hi all

I am trying to upload “you can also use the Serial Console XMODEM transfer to send the FWF file to the router, while connected via Serial Console. From the Bootloader menu it’s possible to upgrade the firmware with this method.”
Is RB1100ahx2 , and cant find the file to upload says that is in the npk but i cant open it.

Thanks on advance

Alberto