如何修复此错误：“E：某些索引文件下载失败。它们已被忽略，或改用旧文件”？

Question

当我尝试在 Ubuntu 13.04 中运行以下命令时：

sudo apt-get update

我收到以下错误：

W: Failed to fetch http://ppa.launchpad.net/upubuntu- 
     com/xampp/ubuntu/dists/raring/main/binary-i386/Packages  404  Not Found

E: Some index files failed to download. They have been ignored, or old ones  used
     instead