Change the shape of an object?

I have a caption with rounded edges, but I'd like to change it to square edges but it doesn't seem there is a way to do that without redoing the whole object.

I can right click and go into "Size and position" or "Format shape" but I don't get the option to change the shape in there.

If I select the object then go into "insert" and "shape" or "caption" all it does it let me create a new shape.

Am I missing something here?