I need this function for adjusting scatter brush but I cannot find any.
Hi An Elephant,
You should find various controls for the scatter brush stroke under the Stroke Options panel. Once you've made the stroke, select the stroke and click on the icon indicated in the screen-shot, and change the 'Pressure' drop-down to 'Fixed', or 'Random' or whatever you prefer. Turn on the Preview checkbox and you will be able to manually adjust the stroke to your liking.
Hope this answers you question!
It's not a feature of the brush stroke, it's a built-in feature. The program modulates parameters while you draw and then applies them, but once that has happened there is no persistent data to modify.