Permission needed to change icons of native Apple apps
Turn off your Mac.
Boot it in Recovery Mode by holding down ⌘-R until the Apple logo with a progress bar pops up.
Click "Utilities" in the menu bar, and select Terminal.
Type the command "csrutil status" and press Enter. 
If enabled, type "csrutil disable".
Press Enter.
Click the  in the menu bar and select Restart. 
Now, the copy and paste method should work.


For more detailed instructions, check out http://hashbangstar.net/blog/changing-icons-in-el-capitan

You can also download a program called Liteicon from 
http://www.macupdate.com/app/mac/24764/liteicon

You still have to do steps 1-8 but it makes changing icons super easy! You can also easily change the Finder icon and Trash Can by using the drag and drop method.