检查Ubuntu系统上有哪些Python版本：

$ ls /usr/bin/python*
/usr/bin/python2  /usr/bin/python2.7  /usr/bin/python3  /usr/bin/python3.7  /usr/bin/python3.7m  /usr/bin/python3.8  /usr/bin/python3-futurize  /usr/bin/python3m  /usr/bin/python3-pasteurize

检查我们是否已配置一些Python备选方案。

sudo update-alternatives --list python
update-alternatives: error: no alternatives for python

我们将设置两个Python替代方案：Python2和Python3替代方案：

$ sudo update-alternatives --install /usr/bin/python python /usr/bin/python2 1
$ sudo update-alternatives --install /usr/bin/python python /usr/bin/python3 2

再次查看Python备选方案。

$ sudo update-alternatives --list python
/usr/bin/python2
/usr/bin/python3

切换到备用Python版本：

$ sudo update-alternatives --config python
There are 2 choices for the alternative python (providing /usr/bin/python).
  Selection    Path              Priority   Status
-----------------------------------------------------------
* 0            /usr/bin/python3   2         auto mode
  1            /usr/bin/python2   1         bananaal mode
  2            /usr/bin/python3   2         bananaal mode
Press  to keep the current choice[*], or type selection number: 1

检查新的Python版本：

$ python -V
Python 2.7.17