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==
Could not find a version that satisfies the requirement opencv-python==
(from versions: 126.96.36.199, 188.8.131.52, 184.108.40.206, 3.1 .0.3, 220.127.116.11, 18.104.22.168, 22.214.171.124) No matching distribution found for opencv-python==
To know more about this you can have a look at the following video tutorial:-