You can pause a session by pressing the pause button within the program screen.
Then resume it by pressing the play button in the same location.
If you want to end the session, click on the back arrow at the top left of the screen and choose from the options that appear: Save and exit / Quit / Resume session
If you choose "Save and Exit" you can comment on the session and save it. You can find this session in the MySessions screen, located in the drop-down menu.
It is also possible to restart a program from the summary of a past session (video below)