Let say you upgraded to Mac OS X Yosemite and realized that you liked the Finder icon from Mac OS X Mavericks more.
You will need to change the icons in a few locations:
For icons that appear in the dock:
/System/Library/CoreServices/Dock.app/Contents/Resources/finder.png
/System/Library/CoreServices/Dock.app/Contents/Resources/finder@2x.png
For the Finder.app icon:
/System/Library/CoreServices/Finder.app/Contents/Resources/Finder.icns
