thank u very much ScruffR
i used particle flash --usb firmware.bin (=DFU Mode, preferable)
i got this reply and running another app successfully
Opening DFU capable USB device…
Run-time device DFU version 011a
Claiming USB DFU Interface…
Setting Alternate Setting #0 …
Determining device status: state = dfuIDLE, status = 0
DFU mode device DFU version 011a
Device returned transfer size 4096
DfuSe interface name: "Internal Flash "
Downloading to address = 0x08080000, size = 12780
Download [=========================] 100% 12780 bytes
File downloaded successfully
Invalid DFU suffix signature
A valid DFU suffix will be required in a future dfu-util release!!!
but what precautions should be take when we use SYSTEM_MODE(MANUAL); as device was not connecting
to network in earlier app?