When following the Installing TensorFlow for Windows guide https://www.tensorflow.org/install/install_windows, after executing
C:\\> pip3 install --upgrad
The issue is your path in the cmd is not that of where your python scripts are placed. In this case you can either navigate to "AppData\Local\Programs\Python\Python36\Scripts" in your terminal and then run the command or you can simply put C:\Users\Your User Name\AppData\Local\Programs\Python\Python36\Scripts to Path variable.
Then re-launch your cmd and type "pip3 install tensorflow" and see the feel the happiness :)