iPhone X and later
From the Home Screen, swipe up from the bottom of the screen and pause in the middle of the screen.
Swipe right or left to find the app that you want to close. Swipe up on the app's preview to close the app.
iPhone SE, iPhone 8 and earlier, and iPod touch
Double tap the home button
(the round button at the bottom of your device) to bring up the multi-tasking view, then swipe the screenshot of the Lifesum app
with one finger to close it. Tap on your home button to get back to the general view. Lifesum is then completely closed.
Please read more on Apple's support page: https://support.apple.com/en-us/HT201330