一般遇到这种问题，就跟墙有关啦，需要更换源。
用清华的源就非常好，去清华镜像的官网看一下说明，https://mirrors.tuna.tsinghua.edu.cn/help/homebrew/
$ cd /usr/local/Homebrew
$ git remote set-url origin https://mirrors.tuna.tsinghua.edu.cn/git/homebrew/brew.git
$ cd /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core
$ git remote set-url origin https://mirrors.tuna.tsinghua.edu.cn/git/homebrew/homebrew-core.git
$ brew update
即可
复原
$ cd "$(brew --repo)"
$ git remote set-url origin https://github.com/Homebrew/brew.git
$ cd "$(brew --repo)/Library/Taps/homebrew/homebrew-core"
$ git remote set-url origin https://github.com/Homebrew/homebrew-core
$ brew update
"$(brew --repo)" 这里填/usr/local/Homebrew