The Core is no longer supported, however these instructions may help.
Since the Particle CLI does not support the Core anymore, you need to manually roll back to v2, see the instructions here.
Put the Core in DFU mode (blinking yellow)
particle flash --factory tinker
Wait till the Spark Core’s main RGB LED returns to flashing yellow
particle flash --usb cc3000
Wait till the Spark Core’s main RGB LED returns to flashing yellow
particle flash --usb tinker
If the device does not go into listening mode, hold down the MODE button until the LED blinks dark blue. Then set the Wi-Fi credentials:
particle serial wifi