1.
Click on the 'Office' button and choose the 'Excel Options' button toward the bottom of the menu.
2.
When the 'Excel Options' dialog box opens, make sure that the 'Popular' tab on the left is selected. In the center of the window you will see the font options. You can select the options you would like to become your new defaults.
3. When you have finished making your selections, click the 'OK' button on the bottom of the dialog box. Your choices will now be the new default font.