Modifying A Tracked File

Once a file has been committed, Git tracks it. The next time you edit it, git status shows it as modified rather than untracked.

git status --short
 M hello.txt

The M in the second column means "modified in the working directory but not yet staged". You stage modifications with the same git add command:

git add hello.txt
git status --short
M  hello.txt

Notice the M moved to the first column. The two columns are staged (left) and working directory (right). Once staged, you commit the change like any other:

git commit -m "Update hello.txt"

Each commit captures a complete snapshot, so you do not lose the old version. You can always look back at it.