Understand that some apps just need to run in the background. Which is the app in question? Some apps run in the background briefly while looking for updates/updating. Others, like anti-virus or VPN need to always run to protect you. If an app is running in the background and using a lot of battery resources, put the app to sleep in the background usage section of the battery settings. Otherwise, follow as per below.
Open the task changer. If you have any apps open, you can select, "close all", or just swipe away the one you want closed. In the upper left of that screen is a bubble showing how many active apps are running in the background. Tap that bubble to see which apps. Next to any app that is not required to run in the background, you will see a "stop" button. Tapping the button will immediately stop the app.
