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
Go to Settings in your phone, choose Applications, and then Manage Applications. Select the Lifesum application, then tap on Force stop. Lifesum is then completely closed.