You can open Calc (Start menu > LibreOffice > LibreOffice Calc), and click File > Open to open an xls or xlsx file but you probably want to associate those file types with Calc.
You can associate file types to LibreOffice in a couple of ways:
- Click Start Menu > Settings > Apps > Default Apps > Set defaults by app > LibreOffice 6.4 > Manage. Click on .xls (and .xlsx) and select LibreOffice Calc as the default to open. Close window when complete
- In File Explorer right-click on an xls file, select from the context menu Open With. A dialog box will appear, click on the link more apps, if you can see LibreOffice Calc in the list then tick the box Always use this app to open xls files, select LibreOffice Calc and press the button OK. If you can’t see LibreOffice Calc in the list then at the bottom there is link for look for another app on this pc, navigate to scalc.exe (probably C:\Program Files\LibreOffice\program\scalc.exe), OK, OK. Repeat for .xlsx.
[Edit 2021-02-28 23:06 UTC - changed from LibreOffice to LibreOffice Calc in Step 1 as default to open]