There are a few ways to reset player progress in a game. Which to choose may depend on your scenario with players, devices, etc. But guessing from your description, I think the simplest will work for you. If not, please write back and I'll suggest other ways.
The simplest way to reset a player's progress is to go the the hamburger menu (three horizontal lozenges in the top left). At the bottom of that menu, "leave game". Then, when you go back in choose "reset" instead of resume.
This will not affect notebook content.
If you need a fresh game including notebook, you will need to log out (at the bottom of the hamburger menu when you're already in the game picker after leaving a game), and log in as a new player.