How can I extract the *.icns from the Get Info dialog?

Previously, I've changed the icon of an application before by manually replacing the icon file .icns located in loremipsum.app/Contents/Resources/loremipsum.icns.

This time, I've downloaded some beautiful icons which are folders with the needed icon and not simple .icns files.

How can I get access of the the *.icns file?

You can mark it in the Get Info dialog, go Edit → Copy, switch to Preview, and select File → New from clipboard.

There's no need to edit a resource bundle. You can copy the icon from an existing folder, file or application by selecting it and choosing Get Info from the Finder File menu or by using the keyboard shortcut ⌘ + i.

Select the icon file in the upper left hand corner by clicking on it once. The copy the file. I use ⌘ + c for this. Now Get Info for the application you want to use the icon for. Select it's icon, and paste the new icon into the application using ⌘ + v.