You can't change the actual font on your iPhone, but you can change the font size in a couple of different ways, especially ...

To enable this, go to Settings, Accessibility, Touch and select Back Tap. This feature turns the back of your iPhone into a ...

You can change your font size in Signal on the secure-messaging platform's desktop, Android, and iPhone apps.

Apple says the flaw involved 'a maliciously crafted font' that could cause an app to shut down or corrupt the process memory.