1. 不要卸载Ubuntu自带的Python2.7和Python3.5，直接安装新版本的Python，然后修改默认的Python软链接即可。

1.安装Python3.8

1) 安装依赖包

$ sudo apt update
$ sudo apt install software-properties-common

2）添加 deadsnakes PPA 源

$ sudo add-apt-repository ppa:deadsnakes/ppa
Press Enter to continue

3) 安装 python 3.8

$ sudo apt install python3.8
报错：
E: Unable to locate package python3.8
E: Couldn't find any package by glob 'python3.8'
E: Couldn't find any package by regex 'python3.8'
  • 解决方法：
$ sudo apt-get update
$ sudo apt install python3.8   //成功安装
$ python3.8 --version
Python 3.8.9

2.采用update-alternatives 切换版本

2.1 查看update-alternatives的帮助信息：update-alternatives --help

2.2 查看有没有python可选项：update-alternatives --display python

2.3 如果没有，输入下面的命令建立python可选项

sudo update-alternatives --install /usr/bin/python python /usr/bin/python2.7 2
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.5 1
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.8 3

3.4 切换python版本：sudo update-alternatives --config python

There are 3 choices for the alternative python (providing /usr/bin/python).
 
  Selection    Path                Priority   Status
------------------------------------------------------------
  0            /usr/bin/python3.8   3         auto mode
  1            /usr/bin/python2.7   2         manual mode
* 2            /usr/bin/python3.5   1         manual mode
  3            /usr/bin/python3.8   3         manual mode
 
Press <enter> to keep the current choice[*], or type selection number: 3