You can control the z-order of shapes using the control for each shape, you should do the following:
- Select the shape you want to bring forwards or send backwards
- On the contextual toolbar that appears near the selected shape click the "Order objects" button and then select the z-order movement of the object that you would like to perform (As shown in the screenshot below)
This also works for groups of shapes with the z-order button appearing when the group is selected. Be aware that with container shapes (the shapes that can contain and hold other shapes ie. the web browser mock-up object the ordering isn't straightforward as the container needs to contain the other shapes.