what have you tried so far?
1.
Open a new Excel spreadsheet. When it opens as a restored window(small), place your mouse cursor to the corners of the Excel window and drag to the ends of the screen to show Excel in full screen.
Close the Excel window and now try to open Excel and check.
2.
Open an Excel file. Maximize the window. Holding down the Ctrl key, close the window. and check by open excel normally.