Method 1
The easiest method of changing your icons is with Pictogram. This way your icons stay modified even after updating them. But this does not work for System and App Store apps.
Method 2
If Pictogram does not work there is another way to change your app icons:
1. Right click on the app in the dock → Options → Show in Finder.
2. Press ⌘I (Or right click → Get Info)
3. Drag the new icon onto the small icon in the top left of the Info window.
4. Done! Keep in mind that this will reset with every app update.