There are three ways you should know to enter DFU mode.
The first, as mentioned before, is to push down the bootloader button and power on your board.
The second method is to be used on boards that don’t have a bootloader button (which is increasingincreasingly rare). You will need to bridge two bootloader pads on your flight controller to enter DFU mode. Check the specific diagram for your flight controller to find out exactly what pads you need to bridge.
The last method is to enter the CLI and type either ‘DFU’ or ‘BL’, which will then put the FC into DFU mode. Obviously this method won’t work if you’re entering DFU mode because you ant connect to your computer.