Modifying commits that did not receive push

Yes!

You can undo your last local commit (without pushing), returning to the previous state of the commit and without losing any changes made to it.

Just do it:

git reset --soft HEAD~1

Being 1 the number of back commands you want to undo. Since you only want the latter, you will use 1 (one).

After this, your pre-commit changes will all be shown as unstaged in git status and you will need to add them again with git add . .

Now just add the file you want and commit again by changing its message.