The only way that I know of doing this is to write the text somewhere else then use Excel's Camera.
The Camera toolbar button is not present by default. To get it, Tools| Customize, click on Tools in the left hand pane then scroll down in the right hand pane to find it. Left click on the Camera and drag it onto a toolbar.
Next, select the cell containing the text and click the Camera button. Navigate to where you want to display the upside down text, left click and draw out an area for display (just like adding a Rectangle drawing object). Click on the green handle on the top of the displayed camera image and drag it round to invert the text.