Depending on the phone model you are using:

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.

OR

Go to the bottom bar, click on the menu button (three lines), swipe between the open windows (left / right) and then swipe up over the window you'd like to close.

Please read more directly here: https://support.google.com/android/answer/9079646?hl=en