How to change Python version of existing conda virtual environment?

Activate the relevant environment, then install your target python version.

conda activate my_env
conda install python=3.6

Rebuild a new environment, for example called myenvi

conda create --name myenvi python=3.6

And make sure the version by

python --version

After installing all packages, double-check with

conda list -n myenvi

Adding to the answer above

conda activate my_env
conda uninstall python
conda install python=x.x