That depends on a couple of questions...What header are you using? Does it have header height sensors on it?
The most likely answers are:
Press the "Float" icon on the cornerpost to turn it off...(this is the side view of the head with the squiggly line under it). If the head has no height sensors, also turn off the AHC button (icon with header only)
There are a number of advanced settings in the commandcenter, the simplest way to adjust the HHC is to tap the 'rabbit/turtle/header' button on the armrest. Tap it twice to get to AHC sensitivity.