Change the default version of Python in TLJH from 3.7 to 3.6

I’m actually trying to import ROOT into a python3 notebook on a Ubuntu 18.04 server provided with TLJH. Since ROOT can only be built using python 3.6 right now, and the default python version of TLJH is 3.7, I’m not able to execute import ROOT properly.
How can I “downgrade” to 3.6 the default python version of TLJH?

EDIT: for the sake of clarity, if I open a terminal from the JH web page, and I import ROOT after having typed “python3.6”, everything works. My question is related to the notebook, which is by default py3.7