Adding band to a product


I want to do some operations using numpy on a single band of a product, using snappy. I get the band with b2 = np.array(product.getBandAt(2)). I do the operations with numpy and now I would like to add the changed band to the product. But I do not know how to do it.

I am very new with snappy with absolute lack of knowledge of java, so any help with the steps to follow will be really appreciated.