It depends if you use the old grass package or the foilage package -> this is the new one:
In the new one everything is controlled with material instance constants. So you just have to open such a material and then you should see some sliders/parameters that you can change. When you dont have any experience with the material editor, I would recommend you to do everything in those MIC’s -> otherwise take a look at the real master materials to see how everything was made 