In the layers panel, click on the little link icon between the two objects of the mask group to unlock it. That makes the individual objects editable while still preserving the grouping. Once unlocked, click on the text object, and then select the Text tool. Click on the text object to highlight it and make your changes. Note that the text editing features will not show up in the Properties panel. They will be accessable via the Text menu on the main toolbar, however. Once you are satisfied with your changes, click between the mask objects in the layers panel to lock them again.