Changing Pl/Python default interpreter

Hi,

I'm running PostgreSQL 10.6 (Ubuntu 10.6-0ubuntu0.18.10.1) with Pl/Python.
Everything is working fine but I would like to change the default
interpreter used by Pl/Python (without recompiling Postgre from scratch).
The docs say that if the PYTHON environment variable wasn't set during
compilation, Pl/Python use any version it found. I wrote a small script and
I found which interpreter Pl/Python is using by default. Now I would like
to modify the symlink it uses to run Python -- I'm just assuming it's a
symlink but maybe I'm wrong. The problem is that I tried to modify all
python executables in /usr/bin but it didn't change Pl/Python default
behavior. Does anyone know where I can find which file Pl/Python use to run
Python? Is there a better approach to change the default interpreter?

Many thanks,

Colin