Note: More than 5,000 icons have been ported from MacOS and Windows to BeOS format, and can be downloaded from Miami Bug. You can also download their IconChomper utility to convert your own icons over.
BeOS icons aren’t stored as separate files, as they are in Windows. Instead, they’re stored as attributes of individual files or filetypes.
To change an icon, you can either drag and drop or copy and paste from any icon well to any other icon well. Icon wells are found in filetypes panels, which live in any of these four places:

• Right-click a file and choose Add-Ons | FileType
• Launch the FileTypes preferences panel and navigate to a particular filetype
• Open an application’s FileType add-on and you can change either the app’s own icon or the icons for all the filetypes that app can handle.
• Open the Tracker in QuickRes to find „permanent“ icons for the Trash, etc.

Where this can get tricky is when a filetype is directly associated with an application. For example, if you try to change the icon for plain text files from the FileTypes preferences panel, you’ll find that your changes don’t make any difference. This is because text/plain is probably associated with StyledEdit or Pe or Eddie, etc. In that case, open the FileType add-on for that text editor, navigate through the list of the filetypes it handles, and select the one you want to change. You can then drag and drop into that icon well.
To edit icons, double-click any icon well to access Icon-o-Matic, where you can touch up the existing icon, paste in existing icons, or create new icons from scratch.
Note: If you change an icon for a global filetype, there may be no easy way to get the original back. If you get into this situation, see the tip Recover Lost Icons.