To edit the shape itself including the label and properties of
the shape, right mouse on the icon in the stencil pane
and select Edit Master > Edit Master Shape. The shape
image will be displayed.
Right-click the shape image and select Show ShapeSheet
(you must be in the developer mode to do this).
In the ShapeSheet, look for the LockGroup property and change
the value from 1 to 0 to unlock the group.
Right-click the shape image again and select
Shape > Ungroup.
Now you can edit the image including the text in the image.