Please note, these steps require Montserrat is installed first.
To change the default font for incoming and outgoing messages:
Select Outlook > Preferences > Fonts.
In the Default fonts for composing messages section click the Font button next to the composition style you want to edit. For example, click Font next to New mail to select a default font for new messages, or next to Reply or forward to set a font for outgoing messages.
On the Font tab, choose the default Font (Montserrat), Font style (Regular), Size (9), Colour & Underline, and Effects (Leave as defaults or Automatic).
Here is a video tutorial you can follow: