You should now be able to edit the text of the object. Alternatively, if you want it to be editable outside the Slide Master you would need to add a new placeholder and remove the existing one as changing the object directly to a placeholder isn't currently possible as far as I know. This can be done by clicking on Insert Placeholder on the Slide Master ribbon, and using the Format Painter to copy the style into the new placeholder. Afterwards, you may have to resize it accordingly, and finally delete the previous one.