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What icon do you want to change to? Icons are generally kept in program files, EXE COM or DLL files.
If you want to find the program files, in the Start menu, right click on any Office application "tile" > select "More" > Open file location. This opens the folder that contains the start menu tiles. Right click on the Excel shortcut > select "Properties". In the Shortcut tab, look in the Target input. This lists the exact path to the program file, ie "C:\Program Files (x86)\Microsoft Office\root\Office16\EXCEL.EXE" (your path may vary)
You can point to other locations to get other icons ...