Afraid you can't change the font on the actual button, that's controlled by Meta to keep the user experience consistent.
The workaround is to ignore the default CTA button and design your own call-to-action as part of your ad creative. Use your own brand's fonts and style in the image or video, maybe add an animated arrow pointing down. This gives you complete creative control and to be honest, it's a much better practise as it looks more bespoke and less like a generic ad.
The native "Learn More" button will still show, but your custom-designed one will be the main visual cue for the user.
Hope this helps
