It's hard to say for sure. For one thing I no longer use Microsoft office.
It depends on the picture though. It's probably just that. A picture, which means the text is part of the picture. It can't be seperated by normal means.
You could try saving the picture then use a graphics editor to recreate the picture, touch up the text box area to try to recreate the original picture, save it then add a new text box. Replace the picture with the new picture.
Photoshop, Paintshop Pro or maybe the Gimp can be used. I've used all three in the past.
If possible just use the graphics editor with an untouched copy of the image if you can obtain it. It's easier.