Remove from stage (unstage) (restore to modified)

$ git restore --staged
$ git reset HEAD

Unstaged changes after reset:
M       README.txt

$ git status

On branch master
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

      modified:   README.txt

Untracked files:
  (use "git add <file>..." to include in what will be committed)
no changes added to commit (use "git add" and/or "git commit -a")

$ git reset HEAD will reset all the files currently staged. See also the unstage alias we created earlier.