To Close a Notebook (Scroll Down for how to open a Notebook)
Right Click the Notebook in the list on the left
And Select the Close This Notebook option
To Open a Notebook that has been PREVIOUSLY OPENED
Right Click below your current open Notebooks
And Select the Open Notebook... option
From here you can see your Recent Notebooks
Click them to Open
To instead Open a Notebook that you have NOT PREVIOUSLY OPENED
Go to the Class Notebook on Teams
Then Select the File option on the Top Bar (1)
And in the Info tab (which should be automatically open) Click Edit in Desktop App
.