On Ubuntu 16.04, there is a version of Python installed by default.
You should not uninstall it or it will break your machine.
Install Python 3.6
sudo apt-get install software-properties-common python-software-properties
sudo add-apt-repository ppa:jonathonf/python-3.6
sudo apt-get update
sudo apt-get install python3.6
Now you can use python 3.6. Check the version by using
python3.6 -V
Install pip for Python 3.6
curl https://bootstrap.pypa.io/get-pip.py | sudo python3.6
Now pip is installed for Python3.6 and can be used to add package to this interpreter.
Install package for Python 3.6
sudo python3.6 -m pip install Flask
My snippets 