You can use a filebrowser like the other answer suggests or an inputbox but both are anoying for the user, you could refer to just the filename without path if you put the excel file in the same map as the script, you can put this in a configuration file that you load in your main script or page or last you could give it to your script as a parameter which you can give in a console command or in a shortcut. let me know which you prefer, then i can give you an example.
EDIT: as promised, actually in the case of Excel it is a bit harder but not impossible, see this example
Set objExcel1 = CreateObject("Excel.Application")
objExcel1.visible = true
strPathExcel1 = CreateObject("Scripting.FileSystemObject").GetAbsolutePathName(".") & "\GE_Wing_To_Wing_Report.xlsx"
Set objWB = objExcel1.Workbooks.open(strPathExcel1)