I would recommend IcoFX which by the way I am currently using.
Goods news is that it is portable, and can be downloaded as a .paf file.
IcoFX is an award winning freeware icon editor. It is an
all-in-one solution for icon creation, extraction and editing. It is
designed to work with Windows XP, Windows Vista, Windows 7 and
Macintosh icons supporting transparency.
Other features
Add, change or delete icons inside exe files
Support for Vista/7 icon with PNG compression
Create icons for Windows 98 / ME / 2000 / XP / Vista / 7
Create icon libraries
Easily convert images to icons
Support for transparency (alpha channel)
Basic usage, to change the icon of an executable;
open the icoFX program and go to the Tools tab and click Resource Editor
In the windows that comes, select open and navigate to the path of your executable
After selecting the executable, right click on the program's current icon and choose Change

- Then choose your preferred .ico.
Hope this helps