To change the text typography, click on a text overlay to see the generic style adjustments menu. Here you can change the font, size, line height, letter spacing, colour, alignment and background colour. These options will be applied to all the text that is in the box.
To add styles specific to sections of the text, double-click on text and an extra style menu will appear. Here you can make the text bold, italic, underlined, create a text link or a bullet point. These options can be applied to different words in the same text box. Simply highlight the section that you would like to add the style too.