Replace Header Text

Out of the box, GeneratePress has a text header title. For reference, it's the text that happens within the place "main-title". (I'm not referring to the page title that appears in the browser title bar.) I'd like to dynamically change this text with a php function. I've already written the php function and it works perfectly. The problem is that if I place it in the Before Header Content or After Header Content hooks, it leaves the GP text. How can I replace that text with my php generated text using hooks.