It's not really possible to edit the shapes without a lot of
behind-the-scenes work.
What you could do is drag out more Matrix shapes and hide the labels of the
new shape behind the boxes of the old shape. Just hit Ctrl+Shift+B to send a
shape to the back.
Or, you could just draw new rectangles, and format them the same as the
Matrix shape. Do this like so:
1. Draw a new rectangle
2. Sub-select one of the boxes in the matrix shape.
3. Shift + click on the new rectangle.
4. Hit the Format Painter button in the toolbar. It looks like a broom, and
is just after the delete X icon in the toolbar.
Your new box will have the same formatting as the sub-box in the matrix
shape.
