The options to move a prop behind or in front of another prop or asset can be found under the menu option for Order.
Alternatively, the same option can also be found on the toolbar under the arrange icon, at the top of the studio:
1) Select the prop that needs to move behind another Prop.
2) Right click on the prop and select the Order option. If the prop is already in front, then the option to Move Backward/Send to Back can be selected. If the prop needs to come forward, then select Move Forward/Bring to Front.
3) The prop should move one layer backward and reveal any props that have a higher layer priority. If the prop were brought to the front, then it would reveal itself as the higher layer priority.