I’d strongly recommend to leave that as it is, it ensures that your view always is centered vertically. If you still want to change it, you can do that on the DirectVR page of the vorpX menu. In addition to that you might have to adjust the right stick y-sensitivity on the Input page to your liking.
Keep in mind though that the feature hugely improves comfort, which is why all native VR games work that way. Think twice (or better thrice :)) before you disable it.