pip install downgrade numexpr code example

Example 1: how to pip install a specific version

# At the time of writing this numpy is in version 1.19.x
# This statement below will install numpy version 1.18.1
python -m pip install numpy==1.18.1

Example 2: stackoverflow: install old version of networkx

pip install matplotlib==2.2.3
pip install networkx==1.10