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I recommend buying CandyBar, but if you want to do it manually, choose a file that you want to change the icon of and press CMD+i, then choose the file of the new icon you want to use and press CMD+i. Copy the icns file in the top left of the second info box by selecting it and pressing CMD+C. Select the icon in the top left of the first info box and press CMD+V.
 
I recommend buying CandyBar, but if you want to do it manually, choose a file that you want to change the icon of and press CMD+i, then choose the file of the new icon you want to use and press CMD+i. Copy the icns file in the top left of the second info box by selecting it and pressing CMD+C. Select the icon in the top left of the first info box and press CMD+V.

i have just tried doing it by Candybar ''Apps extras'' tab...and drag/drop the new icons...
But,they just get dropped there and it shows '.icns' file...not the actual icon =(
 
Go to your Applications folder, right click Xee.app and click 'Show package contents'. Here go to Contents > Resources.
Paste all the icons which you downloaded into this folder. And you're done.
:)
 
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