Undo my last commit and split it into two separate ones

When you accidentally committed some changes to your branch you have various possibilities to “undo” that operation and add some more changes. One is to use git amend to change the commit message or add new files. But what we want to take out all of the committed changes again and maybe even split them into separate commits. Let’s see how we can achieve that. Note: This only holds if you did not yet push your changes to the remote repository