how to remove a song on cd rw and replace it?

Sorry, but I'm not quite sure what you mean. If you mean that you burned a CD-RW disk using iTunes and now wish to modify or add to the disk, you can't. You'll have to erase the disk completely using Disk Utility and re-burn it.

If that's not what you mean, please post back and clarify.

Regards.

You can't erase it in iTunes: and it's not possible to erase just one song by any method. You will indeed have to erase the entire disk using Disk Utility.