Run the following

python -m pip install --upgrade tensorflow

Assuming python is working, TensorFlow should get installed (at least the "Validate the installation" step is green).

This will work, if you are facing pip3 or pip is not recognized as an internal or external command issue on windows:

1. From the desktop, right click the Computer icon.
2. Choose Properties from the context menu.
3. Click the Advanced system settings link.
4. Click Environment Variables. In the section System Variables, find the PATH environment variable and select it. Click Edit.
5. A new pop up will open. Variable name will remain Path. We will change the Variable value to the location of the folder where your python scripts folder is located. Find it.

For e.g. I changed its value to C:\Users\rgupta6\AppData\Local\Programs\Python\Python35\Scripts

6. Close all remaining windows. Reopen Command prompt window, and run your pip3 install --upgrade tensorflow command or pip3 install tensorflow command

That is because you haven't setup the environment variable yet.

Follow the steps by @rajesh

I had the same problem and i found his answer helpful

1. Right click on This PC > Select Properties
2. Select Advanced system settings on the left
3. In the dialog box select Environment Variables
4. In the system variables section select path and cllck on edit
5. Select new and enter the path where the python scripts are..

it is mostly in C:\Users[your user name]\AppData\Local\Programs\Python\Python36\Scripts

6. Then ok.. to all the boxes opened
7. Close cmd if it is already open and now try installing tensorflow using pip again like this

 pip3 install --upgrade tensorflow