Q: El capitan | 15' 2.5 GHz new | graphics isn't AMD R9 M370X but Intel Iris pro.how to change?

Hi,

I recently reinstalled my OS X due to partitions problems.

After the installation, I see in "about this mac" that my graphics is Intel Iris pro 1536 MB and not the AMD Radeon R9 M370X as it was before.

In system information I see that I have the AMD graphics in the graphics/display tab, how can I use it? 

Thanks

General info -

Macbook pro 15' 2.5 GHz running El capitan (11.10.5) , bought it a month ago.