To change the Material you need to set up a parameter in your Mass which is easily done. Select your Mass then choose the Edit buttonThen go to the Extrusion Properties where you will see the Material defined as
. Just to the right is a little button to "add a parameter". Select this button and you will get the Associate Family Parameter dialouge.
Select Add Parameter and fill in the Dialouge box as below:Then OK a few times and Finish Family.
If you now look at the properties of you Mass you will have an option to change the Material