Double-click on the Home button is inactive on iOS 10.3 or later versions. There are three ways to open App Switcher: 1) Swipe up from the bottom to the middle of the screen. 2) Use AssistiveTouch (Click on the AssistiveTouch icon → Click twice on the Home button there). 3) Use AssistiveTouch (Click on the AssistiveTouch icon → Choose Device → More → Multitasking).