[Python] How can I upgrade the Python version and packages in a pyenv virtualenv?

I used pyenv and pyenv-virtualenv for managing a Python virtual environment.

I have a project working in a Python 3.4 virtual environment.

So all installed packages (Pandas, NumPy, etc.) are not the newest version.

I want to upgrade the Python version from 3.4 to 3.6 as well as upgrade other package versions to higher ones.

How can I do this easily?

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