切换到国内源

# 替换brew.git:
$ cd "$(brew --repo)"
# 中国科大:
$ git remote set-url origin https://mirrors.ustc.edu.cn/brew.git
# 清华大学:
$ git remote set-url origin https://mirrors.tuna.tsinghua.edu.cn/git/homebrew/brew.git

# 替换homebrew-core.git:
$ cd "$(brew --repo)/Library/Taps/homebrew/homebrew-core"
# 中国科大:
$ git remote set-url origin https://mirrors.ustc.edu.cn/homebrew-core.git
# 清华大学:
$ git remote set-url origin https://mirrors.tuna.tsinghua.edu.cn/git/homebrew/homebrew-core.git

# 替换homebrew-bottles:
# 中国科大:
$ echo 'export HOMEBREW_BOTTLE_DOMAIN=https://mirrors.ustc.edu.cn/homebrew-bottles' >> ~/.bash_profile
$ source ~/.bash_profile
# 清华大学:
$ echo 'export HOMEBREW_BOTTLE_DOMAIN=https://mirrors.tuna.tsinghua.edu.cn/homebrew-bottles' >> ~/.bash_profile
$ source ~/.bash_profile

# 应用生效:
$ brew update

切换到官方源

# 重置brew.git:
$ cd "$(brew --repo)"
$ git remote set-url origin https://github.com/Homebrew/brew.git

# 重置homebrew-core.git:
$ cd "$(brew --repo)/Library/Taps/homebrew/homebrew-core"
$ git remote set-url origin https://github.com/Homebrew/homebrew-core.git

报错

$ brew update
fatal: Could not resolve HEAD to a revision
Already up-to-date.

$ brew update --verbose
Checking if we need to fetch /opt/homebrew...
Checking if we need to fetch /opt/homebrew/Library/Taps/homebrew/homebrew-cask...
Fetching /opt/homebrew...
Checking if we need to fetch /opt/homebrew/Library/Taps/homebrew/homebrew-core...
Fetching /opt/homebrew/Library/Taps/homebrew/homebrew-core...
Updating /opt/homebrew...
Branch 'master' set up to track remote branch 'master' from 'origin'.
Switched to and reset branch 'master'
Your branch is up to date with 'origin/master'.
Switched to and reset branch 'stable'
Current branch stable is up to date.

# 进入这个地址
Updating /opt/homebrew/Library/Taps/homebrew/homebrew-core...
fatal: Could not resolve HEAD to a revision
Already up-to-date.

$ cd /opt/homebrew/Library/Taps/homebrew/homebrew-core            
homebrew-core (master) » ll                                           
total 0
drwxr-xr-x   3 machao  admin   96 12  2 12:14 .
drwxr-xr-x   4 machao  admin  128 12  2 12:49 ..
drwxr-xr-x  12 machao  admin  384 12  2 14:20 .git

# 从新操作
$ git fetch --prune origin                     
$ git pull --rebase origin master              
From https://mirrors.ustc.edu.cn/homebrew-core
 * branch                    master     -> FETCH_HEAD
$ brew update
Already up-to-date.