There are a couple of ways to change the font on Singl.
An easy way is through a Google Font plugin, like one of these:
You'd then apply the font through your child theme's stylesheet, like this:
font-family: Georgia, Times, serif;
You would adjust the list of fonts to match whatever Google Fonts specifies there. Backup fonts and generic fonts are always recommended - those get listed after the first one, separated by commas.
Another way to change a font in your child theme is manually. It's a little more complicated, but it's another alternative. My colleague James wrote up a good guide here: