Edit > Fill and set to White and Mode: Behind.
If you want to change the background you could go the other way around:
Add the appropriate Layer Mask to the Layer (maybe based on the blue Channel or use whatever fitting Selection Tool you are comfortable with) and add a Solid Color Layer (Layer > New Fill Layer > Solid Color) behind it.
you need to isolate the cut the object from the background first,
there are many ways to delete the White background :
1- use the Quick selection Tool , then refine the edges using select -> select and Mask ( or refine edges if you are not on the latest CC 2017 ) then you may press the delete button or use the layer mask
2- select -> color Range , select the white
3- use the magic wand
once you have the Transparent background, you can use any background color by creating a New solid Fill
below are screenshots to demonstrate that :
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Does something like this help:
The way I did is to first delete the white background using Magic Wand and then create a new Layer and move it at the bottom of the pile and use Fill Layer feature on this new layer to create a new color for your background.
Hope you get the idea.