Most Jupyter users will never need to install Jupyter Server manually. Jupyter Web applications will include the (correct version) of Jupyter Server as a dependency. It’s best to let those applications handle installation, because they may require a specific version of Jupyter Server.
If you decide to install manually, run:
pip install jupyter_server
You upgrade or downgrade to a specific version of Jupyter Server by adding an operator to the command above:
pip install jupyter_server==1.0