rpi Bfgminer clockspeed problem

[olliedog12000]

Hi all can anyone point me in the right direction on how a adjust the clockspeed in Bfgminer. I have it running a rockminer r-box2 100ghs but the speed is set to 270 but I know I can run it at 350.

when i go into settings to change it i says to use "unsafe:n" to overclock other than just typing 350:n and pressing enter whilst on the same screen
 I can not see any other way to do it.  I am losing a lot of hashing power as at the moment it is only averaging at 60ghs and not the 120ghs i know it can.