I found how. I went to C:>Windows>Installer>{9028040C-6000-11D3-8CFE-0050048383C9}> (replace > by backslash) and then right click on "accicons.exe", choose properties and uncheck read only box. I did that with "fpicon.exe" "outicon.exe" "pptico.exe" "wordicon.exe" "xlicons.exe". And then, I started Microangelo librarian, then opened the file "Microsoft Excel.ink". Then, i saw a lot of icon, i right clicked on the one i wanted to change and choosed replace. I browsed to the icon i wanted and then i clicked open. I choosed save and the icon is changed. I didn't reboot but i will and post an other message to let you know if it worked fine.