Dell 9478N Owner's Manual page 97

Steps
1. Remove the screw (6-32) that secures the front I/O bracket to the chassis.
2. Rotate and remove the front I/O-bracket from the chassis
3. Disconnect all the cables that are connected to the system board.
4. Remove the seven (#6-32) screws that secure the system board to the chassis.
Rimozione e installazione di unità sostituibili sul campo (FRU) 97