I don't believe there's a way to change this in software. When a value is specified in a datasheet with Min, Max, and Typical values, this usually means that the device will always aim for the Typical value, but it is guaranteed to fall within the min and max values. Since the typical value is explicitly stated on the datasheet, it is unlikely that this value can be changed. An offset voltage of 1.25V is part of the typical LVDS standard. Is there a particular reason that you need to change it?