How to change default font
To change the default font, follow these steps:
1. On the File tab, click the Options button:

2. In the Excel Options dialog box, in the General tab, under When creating new workbooks, choose the font in the Use this as the default font list, and specify the font size in the Font size box:

Note: To begin using the new default font and font size, you must restart Excel. The new default font and font size are used only in new workbooks that you create after you restart Excel; existing workbooks are not affected.
See also this tip in French: Comment changer la police par défaut.