Drag the mouse pointer over the module you want to change. The module is marked with a dark frame with a title bar at the top. It tells you what kind of module it is, and there appears a pen icon to the right for editing.

Click on the pen icon  to get to the editor. It opens an editing tool, which is adapted to the different module types. For a text module a text editor will open that are known from most computer programs. There you can edit the text content, format it, insert links and images, choose font style, etc.