In some circumstance, you will need .xml file to be opened in Excel. The following are how you can set Excel to be default app to open .xml files.
1. Run regedit.exe
2. Go to HKEY_Current_User\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.xml\OpenWithList\
3. Change the value of b to EXCEL.EXE
4. Restart the computer
Note: Please ensure you have Office installed.