Amend a Git Commit

If you left out some files from a commit or modified some files after a commit, and it’s more appropriate to include these changes in the currently staged commit rather than making a new commit, you can amend the commit, adding these extraneous files.

First stage the files you wish to add to the commit:

git add [file1 file2 ...]

Then add them to the current commit thusly (you have two options):

# with the existing commit's message

git commit --amend -C HEAD

# or with a new message:

git commit --amend -m "Your message."

FYI, the -C option is documented as follows:

-C <commit>, --reuse-message=<commit>
    Take an existing commit object, and reuse the log message and the authorship information (including the timestamp) when creating the commit.