By modifying some values on human's M2 model itself. Change will affect all head and shoulder items.
Or, in helmet's case, you can just change M2 model of helmet which is used by human. That will affect only one helmet and only on human. The same can't be done with shoulder items though, those would get scaled for all race/gender combinations. So it depends on what you want to do. This is short list of your 2 only possibilities.