The steam app default icons are inside the .exe file (steam.exe), 
which you can't access the icons manually!

If you changed your steam default icon and want to change back to the 
defaul steam icon, follow this steps:
  1. Right click Steam in the Task Bar.
  2. Right click Steam and click Properties.
  3. At the bottom, click Change Icon.
  4. On your current location path, paste "C:\Program Files (x86)\Steam\steam.exe"
and hit Enter.