Error "Unable to open Jupyter Notebook: Port 8888 is already in use"

The port 8888 is used almost exclusively by Jupyter, and the fact that it's in use indicates more than anything that you have another Jupyter session already running. I'm 99% certain you can just kill the task that's using it, or you can run

jupyter notebook list

to see current notebooks. And as mentioned in the other answer you can run

jupyter notebook --port 8889

To run on a different port instead (replace 8889 by any other number you'd like).

You could use --port argument to instruct jupyter to start the notebook server on another port.

Use jupyter notebook --help command to view the complete list of options.