To do so, you have to first check out which app is using the most mobile data. If your phone is running Android 4.0 and above, you can go to “Settings -> Data usage” to view which app is using the most mobile data.
Once you have identified the app, tap on it and scroll all the way down to the bottom. Check the box “Restrict background data”. This will prevent the app from accessing your mobile data (it will still work in WiFi).
That’s it.