As others have mentioned, you can do this via nbextensions. I wanted to give the brief explanation of what I did, which was quick and easy:
To enable collabsible headings: In your terminal, enable/install Jupyter Notebook Extensions by first entering:
pip install jupyter_contrib_nbextensions
Then, enter:
jupyter contrib nbextension install
Re-open Jupyter Notebook. Go to "Edit" tab, and select "nbextensions config". Un-check box directly under title "Configurable nbextensions", then select "collapsible headings".