When following the Installing TensorFlow for Windows guide https://www.tensorflow.org/install/install_windows, after executing
C:\\> pip3 install --upgrad
This will work, if you are facing pip3 or pip is not recognized as an internal or external command issue on windows:
For e.g. I changed its value to
C:\Users\rgupta6\AppData\Local\Programs\Python\Python35\Scripts
pip3 install --upgrade tensorflow
command or pip3 install tensorflow
command