#1: Do you really mean delete workbook? This option is not available from within Excel. You'll have to do that in OneDrive.
If you want to delete a worksheet:
Right-click the sheet tab.
Select 'Delete' from the context menu.
This option is only available if the workbook has more than one visible worksheet.
#2: In Excel Online, you can only duplicate a worksheet within the same workbook. It does not have an option to move or copy a sheet to another workbook. You'll have to copy/paste all data from the source sheet to the target sheet in another workbook. Or edit the workbook in the desktop version of Excel.