How to put Arduino 2560 R3 in DFU mode ??

i am having trouble connecting my arduino with IDE. My IDE is not detecting it. i have tried multiple cables but still facing the same problem.
I am trying to put it into DFU mode to update/restore the mega16U2 Chip. but it is not going it to DFU mode. :frowning:
PLease anybody help me with it ??