Dfu mode...

Hello, I need help, please. I just built a 3.5-inch drone and I have a SpeedyBee F405 mini. I am trying to update it, and I cannot get it into DFU mode. I have tried using ImpulseRC, the CLI "bl" command, pressing the button on the flight controller, and connecting the USB. Anyone have an idea on how to fix this? Thank you in advance.