if its a basic font like arial, times new roman, monospace, etc, you can use this code:
<style>
h1{
font-family: times new roman !important;
}
</style>
and just change the name to the font you want to use.
if you want to use a font from fonts.google.com or cdnfonts.com you will need to copy and paste the import code they give you and than also copy and paste the font-family code which you paste into the code above.