1. 首先我们要找到国内的镜像源路径
我选择了清华的镜像源，链接如下：
https://mirrors.tuna.tsinghua.edu.cn/help/ubuntu/
打开链接如下图：
接下来按图中提示操作即可，当然也可以自己搜索其他的镜像源。
2. 备份系统自带更新源配置文件
打开终端切换到管理员（修改配置文件需要较高权限，如不切换也可以在每条命令前加sudo，不过个人感觉有点麻烦），进入 /etc/apt 目录，找到 sources.list 文件，进行备份，如下图：
也可以直接用命令：cp /etc/apt/source.list /etc/apt/source.list.backup ，则不用切换目录直接备份。
3.修改配置文件source.list内容
输入命令：gedit source.list 打开文件，把文件内容全部删除，再把更新源路径粘贴进来。
粘贴后如下图：
之后保存，并退出。
4. 更新源
输入命令：sudo apt-get update
开始更新源
更新完成 ：
至此，更新源就已经成功了！
如果使用此软件源有部分问题，推荐使用“推荐软件源.txt”中的软件源，将代码直接粘贴到刚刚打开的source.list文件头部就行了。
推荐软件源1（阿里云）：
deb http://mirrors.aliyun.com/ubuntu/ vivid main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ vivid-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ vivid-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ vivid-proposed main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ vivid-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ vivid main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ vivid-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ vivid-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ vivid-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ vivid-backports main restricted universe multiverse
推荐软件源2：
deb http://mirrors.aliyun.com/ubuntu/ quantal main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ quantal-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ quantal-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ quantal-proposed main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ quantal-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ quantal main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ quantal-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ quantal-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ quantal-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ quantal-backports main restricted universe multiverse
# 默认注释了源码镜像以提高 apt update 速度，如有需要可自行取消注释
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-updates main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-updates main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-backports main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-backports main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-security main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-security main restricted universe multiverse
OK，这样的话，就可以比较流畅的使用ubuntu系统了！