Install Python Specific version upgrade or downgrade and Also Virtual Environment with the specific Python Version

1. install python 3.7 and 3.7 virtual env 

**********************************

sudo add-apt-repository ppa:deadsnakes/ppa

sudo apt update

sudo apt install python3.7

sudo apt install python3.7-venv

2. create environment

*******************

#goto your project root folder

python3.7 -m venv env3.7

source env3.7/bin/activate

now check version, all same

************************

python3 --version

python --version

python3.7 --version

pip --version

pip3 --version

3. upgrade pip

*************

python3.7 -m pip install --upgrade pip