How do I manage Lumia 635 Background Tasks ?

[hfinge]

Hello,
I just acquired a Lumia 635 (France). Updated the systems a few day ago.
I would like to know if there is a way to manage background tasks.
For instance, I would like to know how to turn off Whatsapp background task when I don't wish to use it. Same for other messaging applications, but also for other apps I don't want to be running in the background all the time.
This has to be selective. I don't want to turn ALL background apps OFF at the same time, as the Battery Saver (in Settings) proposes to do.
Any idea ?
Thanks.

 

[switch side method]

Hi..go to settings and select battery saver.from there you can choose what apps to turn off/on