First, open a terminal window. In Windows, for example, press Windows + R, type cmd, and press return.
From the command window, go to the folder where you have download the notebook files, or to any of if ancestors folders (if they exist), by means of the command cd "path". Finally, launch the Jupyter kernel with
jupyter notebook
This should open your default browser with the directory of the folder you went in and then it is enough that
you click on notebook name you want to work with.