The text within the header graphic is embedded in the image and cannot be changed.
However, you can change the header graphic completely by clicking on it and then clicking Select Image on on the left.
You can also add text on top of the graphic by clicking on Edit Image on the left and then click View Advanced Editing Options.
Select Text from the options at the top (you may need to click the arrow to see additional options if you do not see Text).
Select your text color and Font
Hover your mouse over the text box and click and drag to position the text.
Click in the box labeled Enter text here and type your text.
To re-position again click outside of the editing box.