问题情况

无论怎么更新pip，pip依然是老版本

C:\Users\Changxing>python -m pip install --upgrade pip
Requirement already up-to-date: pip in d:\python38_64\lib\site-packages (20.0.2)

C:\Users\Changxing>pip --version
pip 10.0.1 from d:\python38_64\lib\site-packages\pip-10.0.1-py3.8.egg\pip (python 3.8)
解决方案

首先，先确保一下环境变量中的Python环境是否统一，即以下两个路径的Python环境是统一的。

D:\Python38_64\
D:\Python38_64\Scripts\

确认环境变量统一后，执行如下命令（下载最新的get-pip.py，重新安装pip）：

curl https://bootstrap.pypa.io/get-pip.py -o get-pip.py
python get-pip.py

运行结果如下：

C:\Users\Changxing>curl https://bootstrap.pypa.io/get-pip.py -o get-pip.py
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 1764k  100 1764k    0     0  53157      0  0:00:34  0:00:34 --:--:-- 40634

C:\Users\Changxing>python get-pip.py
Collecting pip
  Using cached pip-20.0.2-py2.py3-none-any.whl (1.4 MB)
Collecting wheel
  Using cached wheel-0.34.2-py2.py3-none-any.whl (26 kB)
Installing collected packages: pip, wheel
  Attempting uninstall: pip
    Found existing installation: pip 20.0.2
    Uninstalling pip-20.0.2:
      Successfully uninstalled pip-20.0.2
Successfully installed pip-20.0.2 wheel-0.34.2

检查当前pip版本已经更新，问题解决

C:\Users\Changxing>pip --version
pip 20.0.2 from d:\python38_64\lib\site-packages\pip (python 3.8)