Just to refresh your memory if a program is called File1 then it's icon will be called File1.info the icons are not embedded into the executable like they are with Windows (95+).
If you just want to swap one icon for another then all you have to do is copy one .info file over another for example to change the more icon to the multiview icon you would open a shell and enter copy sys:utilities/Multiview.info sys:utilities/More.info
However I don't recommend doing this as Amiga icons have different types (Disk, Tool, Project, Garbage & Drawer) which will confuse the hell out of Workbench and cause problems with Tool Types, Comments, Stack size, etc.
To actually edit the images and manipulate icon types you will need an Icon Editor, the one that comes with Workbench (in the Tools drawer) is adequate for simple editing but you may want to try a third party editor like Iconian
which is free and more fully featured.