cd /usr/local
    git remote set-url origin git://mirrors.tuna.tsinghua.edu.cn/homebrew.git
    brew update

    #如果速度还是很慢，可以尝试以下操作，然后重试update
    cd ~/tmp
    git clone git://mirrors.tuna.tsinghua.edu.cn/homebrew.git
    rm -rf /usr/local/.git
    rm -rf /usr/local/Library
    cp -R homebrew/.git /usr/local/
    cp -R homebrew/Library /usr/local/

http://heepo.github.io/%E5%B7%A5%E5%85%B7/2015/08/05/Homebrew-Mirror-Links.html [Homebrew镜像源]