Q. How can I change the font of my header block?
A. There's 2 ways this can be done.
The easiest is to update the Body font, as this is the default font for the whole email.
On the builder, in the "Body" tab, change the font to your prefered font. This will update all the text elements to be that font if they're still marked as "default".
The second way is to toggle the "default" font off in the Header Config and replace it with the prefered font: