Are you asking how to make Windows forget which program to use or do you just want to change the icon?
To make Windows forget...
1) Go to "Control Panel" and then "Folder Options".
2) Go to the "File Types" tab.
3) Select the extension you want to change and click "Delete".
4) Click "OK".
Now, Windows will not know which program to use to open the file.
To change the icon...
1) Go to "Control Panel" and then "Folder Options".
2) Go to the "File Types" tab.
3) Select the extension you want to change and click "Advanced".
4) Click "Cange Icon..." and browse to the icon you want. (To use the "unknown file type" icon, type "C:\WINDOWS\system32\shell32.dll" in the "Look for icons in this file" field.)
5) When you have the icon you want, click "OK" in the three windows.
Now, Windows will know which program to use to open the file but it will show the "unknown file type" icon.