In Engrave mode, choose Engrave > Font Styles to open the Edit Font Styles dialog.
Select Default Text Font from the Font style menu.
Select the font family you want from the Font family menu.
Click OK to save your changes and close the dialog.
Choose Engrave > Paragraph Styles to open the Paragraph Styles dialog.
In the paragraph style list, select Default Text.
Select the font family you want from the Font menu.
Click OK to save your changes and close the dialog.
Result
The font family used in the selected default font and paragraph styles is changed. This also affects the font family used by all other font and paragraph styles in the project whose font family has not been overridden.
I am waning to change the font at the start of the music where it might say
To change the font used for tempo indications, you need to edit the Immediate tempo text font, which is the font used for the bold text like “Allegro”, Metronome music text font, which is the font used for the note value in the metronome mark, and Metronome text font, which is the Roman font used for the digits following the note value. These are all found in Library > Font Styles.
Is it possible to edit the Horizontal Line Font for a single type of line, rather than for every type of line? As in, to override the default in a singular instance?
I’m afraid it is not, no, though this is something that has been mentioned before.
You may be able to achieve the result you want by defining a custom annotation using Library > Line Annotations and then creating a new horizontal line style that uses your custom annotation.