Go to Settings > Locate & tap on Data Usage. To stop all apps from running in the background, click on the menu icon on top right & select ‘Restrict Background Data’. Or, locate and tap on the app you want to stop from running in the background & tap on Restrict Background Data.
Go to Settings > General > Background Apps > TOGGLE OFF “Background App Refresh” to stop all apps. If you wish to stop selective apps then only toggle off the desired apps.