You can also use the Info window (RAmiga+I) to replace icons. Just drag the new icon over the one in the Info window, hit save, and you're done. I like this method because 1) it preserves all your positioning/snapshotting settings (in contrast to replacing the entire .info file) and 2) it doesn't require typing exact filenames into the shell.
works great but won't work on a CD Drive as I'm getting 'drive is write-protected' when I click save.
If you want to change/modify the default CD0: icon (the one that appears on the Workbench), that's ENVARC:Sys/def_cd0.info.
If a CD has its own Disk.info, that will override the default icon, however.
checked that cdo.info in envarc and it does shows the CD icon but dammit all the CD's I burned years that don't have any icon.info file on them always show up as a disk but not CD??