如何使用搜索引擎
其实这个问题并不难，我又被坑了。百度搜的东西不靠谱啊，以后这种问题一定要用英文在 Google 或者 Bing 上搜索，这样才能搜到原汁原味的答案。就当是一个教训吧。
搜索 fork sync，就可以看到 GitHub 自己的帮助文档 Syncing a fork 点进去看这篇的时候，注意到有一个 Tip: Before you can sync your fork with an upstream repository, you must configure a remote that points to the upstream repository in Git.
根据这两篇文章，问题迎刃而解！
具体方法
Configuring a remote for a fork
给 fork 配置一个 remote
主要使用
git remote -v查看远程状态。
git remote -v
# origin https://github.com/YOUR_USERNAME/YOUR_FORK.git (fetch)
# origin https://github.com/YOUR_USERNAME/YOUR_FORK.git (push)
- 添加一个将被同步给 fork 远程的上游仓库
git remote add upstream https://github.com/ORIGINAL_OWNER/ORIGINAL_REPOSITORY.git
- 再次查看状态确认是否配置成功。
git remote -v
# origin https://github.com/YOUR_USERNAME/YOUR_FORK.git (fetch)
# origin https://github.com/YOUR_USERNAME/YOUR_FORK.git (push)
# upstream https://github.com/ORIGINAL_OWNER/ORIGINAL_REPOSITORY.git (fetch)
# upstream https://github.com/ORIGINAL_OWNER/ORIGINAL_REPOSITORY.git (push)
Syncing a fork
- 从上游仓库 fetch 分支和提交点，传送到本地，并会被存储在一个本地分支 upstream/master
git fetch upstream
git fetch upstream
# remote: Counting objects: 75, done.
# remote: Compressing objects: 100% (53/53), done.
# remote: Total 62 (delta 27), reused 44 (delta 9)
# Unpacking objects: 100% (62/62), done.
# From https://github.com/ORIGINAL_OWNER/ORIGINAL_REPOSITORY
# * [new branch] master -> upstream/master
- 切换到本地主分支(如果不在的话)
git checkout master
git checkout master
# Switched to branch 'master'
- 把 upstream/master 分支合并到本地 master 上，这样就完成了同步，并且不会丢掉本地修改的内容。
git merge upstream/master
git merge upstream/master
# Updating a422352..5fdff0f
# Fast-forward
# README | 9 -------
# README.md | 7 ++++++
# 2 files changed, 7 insertions(+), 9 deletions(-)
# delete mode 100644 README
# create mode 100644 README.md
- 如果想更新到 GitHub 的 fork 上，直接
git push origin master就好了。