It's very, very simple you just need to...
1. Select the icon you want to change your folder, app or file to.
2. Choose "Get Info" from the File Menu, or click command-I.
3. Click the icon in the upper left of the info window.
4. Choose "Copy" from the Edit menu (or type command-C).
5. Select your folder, app or file on the desktop and "Get Info" for it.
6. Click the icon in the upper left of the info window.
7. Select "Paste" (command-V) from the Edit Menu.
And that should do the trick for you.
I'm not having a go or anything, but a quick search would throw up tons of threads asking the same question, and giving the same answer... 😉