In the process of using git, if you use git add . to submit all the files, but some files do not want to be submitted, this is the undo operation that needs to be used. You can use the git status command to know how to operate. 
Enter this command to undo the unwanted submission. document.
If you want to know more about git undo operations, you can refer to https://git-scm.com/book/zh/v1/Git-basic-undo operations