If it is to replace the icon of an app, show the contents of the app by right-clicking the app and choosing show package contents.
Them choose contents, and resources. There should be a .icns file in there: make a copy of it on the desktop to be sure you can replace it if anything goes wrong.
Then, change the name of the .icns you want to use to the name of the file in the app. Drag it inside the resource folder, and choose replace in the dialog box.
Now launch the app and the icon should change.
If you want to use it on a folder or on a file, open it in Preview, copy it, and paste it in the show info box.
Hope it helps 🙂