How to change the default font used in the text elements of my chart?

Click on an empty area in your chart (i.e., below the x-axis). This opens the general chart properties under the Presentation properties tab.
See here pre-1.26 version view
In the Appearance box, change the default font. You can use the '…' button to select from the full list of installed fonts.

For more information:

Elements of the chart