Different material properties for a set of elements

Hello, I use the pde toolbox to simulate a structure and obtain its frequency responses. Everything works well up to this point, but now I need to change the material properties for a region/element(s) independently from the rest of the body. Is there any way to do so without having to directly edit the matrices?

Thank you!

[ADSW121365]

If you can elaborate on how your code and geometry is setup this maybe possible.