I am using Ultra-flat-orange icon pack, I am satisfied with all the icons but I just don't like the folder icon, is there any way by which I can change only the folder icon (I want to keep the other icons from this pack, just want to change only the folder icon) and the change should be automatically applied to all the folders, not just one folder.
Grab the Unity Tweak Tool, and select "Icons" under the Appearance section.
You should be able to do it from there.
If you want to be able to switch an individual default icon, I read in this Ubuntu Forums thread that the default icon is designated in this file:
Look for the line that reads:
Try adjusting that (to your new icon file) and see if it helps.