Using the Menus...

Open a cloud notebook

Choose File Open from Wolfram Cloud... and open the cloud notebook you want to edit:

Edit the notebook

Edit the notebook as you would any other desktop notebook. When you save the notebook, your changes are saved in the cloud, not on the desktop.

Programatically...

Open a cloud notebook

Open a cloud notebook on the desktop:

Edit the notebook

Edit the notebook as you would any other desktop notebook. When you save the notebook, your changes are saved in the cloud, not on the desktop.