Ubuntu Software Source Update

Open the Ubuntu Software Center and open the software source in the "edit" option.

Select server address: For example, Tsinghua's Mirror address:

Select OK to close, automatically update, update if an error occurs, such as to indicate that an address is not available (this may be discarded), then in the "other software" in the list of the corresponding URL is unchecked. Then continue with the update.

After this step is complete, run in the terminal:

sudo apt-get update   #更新软件源

Ubuntu Software Source Update