Ubuntu replaces Tsinghua source

1. First use the following command to back up the original source file

sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak2

2. Enter Tsinghua University Open Source Software Mirror Station , find Ubuntu, and click help 

3. Replace all the text in "sources.list" with Tsinghua source, save and exit

4. Finally, execute the following command to update apt

sudo apt-get update

If an error is reported in this step, change https in the file to http