When following the Installing TensorFlow for Windows guide https://www.tensorflow.org/install/install_windows, after executing
C:\\> pip3 install --upgrad
Typing the python command before that should do the trick. In my case (on Windows 8.1 with Python 3.6), I had to type 'py' instead of 'python' as follows:
py -m pip install --upgrade tensorflow
The answer depends on the system you're using.