卸载：
1、卸载python3.4
sudo apt-get remove python3.4

2、卸载python3.4及其依赖
sudo apt-get remove –auto-remove python3.4

3、清除python3.4
sudo apt-get purge python3.4
or
sudo apt-get purge –auto-remove python3.4

安装：
下载python
wget https://www.python.org/ftp/python/2.7.9/Python-2.7.9.tgz
wget -P /usr/lib https://www.python.org/ftp/python/2.7.9/Python-2.7.9.tgz(下载文件到指定目录)

解压、编译安装
tar -zxvf Python-2.7.9.tgz
cd Python-2.7.9
./configure –prefix=/usr/local/python-2.7.9
make
make install

系统自带了python版本，我们需要为新安装的版本添加一个软链
ln -s /usr/local/python-2.7.9/bin/python /usr/bin/python2.7.9