Summary
To unstage a single file in Git, enter the following command in your terminal:
git reset HEAD file-name
To unstage all of the files that are currently staged, enter the following command in your terminal:
git reset
Exercises
First, add some code to index.html. Then stage the changes using git add .. Then unstage them using git reset.
References
Undoing Things from the Pro Git book
git-reset from the Git documentation