iPhone X and later

From the Home Screen, swipe up from the bottom of the screen and pause in the middle of the screen.
Then 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