I know of course, I can do it with the camera app on the phone if the "Aperture mode" has been activated. Is it possible to refocus such image once it's downloaded from the camera? How?
Pretty much, no. The Huawei P9's dual cameras are actually capturing two entirely separate images and then combining them, with a bit of software processing, to simulate the effect of adjusting focus. On the assumption that you're downloading a JPEG or similar from the camera, you've reduced yourself to just one image and thrown away the extra information you need do this kind of modification.
If you save the original picture to your computer then the information is still there. Use this tool to refocus on your computer:
Felix's suggestion is more user friendly, but if you want a simpler, command line solution which just separates out the various frames of data you can try: