Step 1: Find an icon
Find your replacement icon (transparent PNG files work best) and copy the image itself using the Edit menu or Command+C on your keyboard.
Step 2: Get the info
Click on the file, folder or app whose icon you want to replace, head into the File menu and choose Get Info or hit Command+I.
Step 3: Highlight the old
In the Get Info window that appears, click on the existing icon in the top-left corner and ensure that it’s highlighted with a blue glow.
Step 4: Paste in the new
With the old icon highlighted, head into the Edit menu and paste in the new icon image you copied earlier on to finish things off.