If you want to change commit message ，用 git commit -amend.

If you want to add some content, don’t touch the last commit(就叫 commit-1 吧)，接着修改，然后提交一个新的 commit (commit-2)。然后 git rebase -i HEAD~~, the following content will appear:

pick commit-1-hash commit-1-message
pick commit-2-hash commit-2-message

Save the second pick 改成 squash and exit, and the following will appear:

# first commit message:
commit-1-message

# second commit message:
commit-2-message

Delete these things and change them to what you want commit messageSave and exit.

Then you can git log 查看一下，两个 commit 已经合并成为新的 commit.

You can refer to:
5. Use rebase -i to merge and submit

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BTW, if you can only commit the last time commit 已经推到远程分支了，而且该分支有人跟你合作，那么最好还是不要修改了。因为你修改之后，就跟远程分支不同步了，所以需要 git push -f origin your-local-branch, this may cause other people's branches to be out of sync with the remote.

git commit —amend

Can be used to undo the last submission

git reset HEAD^