Commit Amend (Git)
In Git, you are allowed to modify the most recent commit. This basically helps if, you for example forgot to add a file or forgot to change a piece of code.
After modifying the files you want to have changed, stage them (or add the -a parameter to the following command) and execute
git commit --amend
This shows you the entered commit message, which can also be altered.
For the workflow TYPO3 uses with Gerrit, it is fine to amend any commit, which is pending in Gerrit. However, in projects where Gerrit is not used, never amend a commit which has been pushed to any external repository and might already be used by other people.