You can easily swap between apps without having to close the one you’re using. You can do this using a multitasking view known as the App Switcher.
To access the App Switcher, double-press the Home button quickly, or swipe up from the bottom edge of the screen and pause for a moment before releasing your finger. This will reveal your apps in the order they were last opened, each represented by a screenshot of the app itself.
Swipe from right to left to see more and tap any of the screenshots to switch back to the associated app.
To force close an open app, swipe up on the screenshot. Close multiple apps at a time by using two or three fingers – simply swipe up using the same motion to close them.