If you want to change the font for the "A" at the head of column A, and the "1" at the left of column 1, Tools, Options, General, Standard Font, and choose a font.
This will then become the standard font for any new workbook you create, and will control all the text that you input into any new workbook. However, if you want a different font for the text within the workbook, you can still change that.