head «for 8» unscrew the two nuts securing the fuse box to the body...
...and remove the blocks.
In order not to get confused, we immediately connect each disconnected wire to the corresponding output of the new block.
To replace only an additional unit...
...with the same head «for 8» unscrew the two nuts securing the additional unit to the brackets and reconnect the wires to the new unit, as mentioned above.