[python] How to downgrade the installed version of 'pip' on windows?

On a windows 7 machine I have pip version 1.5.6 installed:

pip 1.5.6 from C:\Users\dietz\PNC\tas\ENV\lib\site-packages (python 2.7)

In order to find the reason for an error I want to install a different version of pip, which worked fine for me. So how can I uninstall pip and install version 1.2.1 instead?

If downgrading from pip version 10 because of PyCharm manage.py or other python errors:

python -m pip install pip==9.0.1

well the only thing that will work is

python -m pip install pip==

you can and should run it under IDE terminal (mine was pycharm)