To install openCV2 you can do via pip you can specify the package version to install using the following:
pip install opencv-python==2.4.9
However, that package does not seem to be available on PyPI.
A little trick for checking available versions:
pip install opencv-python==
Which returns:
Could not find a version that satisfies the requirement opencv-python==
(from versions: 3.1.0.0, 3.1.0.1, 3.1.0.2, 3.1 .0.3, 3.1.0.5, 3.2.0.6, 3.2.0.7) No matching distribution found for opencv-python==
To know more about this you can have a look at the following video tutorial:-