On a single VI you can simply select all the controls and change the font using the dropdown menu - this will change the font for all selected items.
If you're trying to do this for multiple VIs then you'd have to resort to using the VI server. You can get a reference to the VI's front panel and then iterate over all the controls/indicators. Properties are available to set the font for captions and labels and the control text, if it has any (like numeric controls). Basically, 2 for-loop. One loop for the VI references, and another for the control references.
