When using the Prota Space app on your Android smartphone, clicking on the physical back button on your phone (if there is one) will shut down the app and bring you back to your Homescreen.
To go back to a previous step or a previously open app, use the sidebar.
Tilt your phone from left to right to make the sidebar tab appear. Then click on it to open the sidebar.
With the sidebar, you can either go back to the home screen page or jump to a previously used app