Are you able to move any of the other squares?  You may need to uninstall the software and remove any profiles you have then reinstall.  You should be able to drag the set points around under normal conditions.
 
Did you create this custom fan curve or is it a preset curve?  I've never used this software specifically, but all these custom fan curve editors work basically the same way.  I was thinking that if this is a preset curve, you may not be able to modify it, but if you can move any of the squares on the curve, you should be able then to move the top one.