This should help, although some steps might be a bit dated:
This might also help:
https://docs.particle.io/support/troubleshooting/key-management/core/
You can flash your applications using DFU-util. This works nicely in combination with the CLI. Once you've coded your app in the web IDE, go to the firmware overview, and click the little cloud icon next to the project's name. That'll download a .bin file. If you've got the CLI installed correctly, you should be able to use particle flash -usb firmware.bin
from the directory where the .bin in saved, while your Core is in DFU mode.