How to undo the last Git commit?

http://git-scm.com/docs/git-reset

Undo a commit and redo

$ git commit ...                              (1)
$ git reset --soft HEAD^               (2)
$ edit                                             (3)
$ git add ....                                   (4)
$ git commit -c ORIG_HEAD      (5)
This is what you want to undo

This is most often done when you remembered what you just committed is incomplete, or you misspelled your commit message, or both. Leaves working tree as it was before "reset".

Make corrections to working tree files.

Stage changes for commit.

"reset" copies the old head to .git/ORIG_HEAD; redo the commit by starting with its log message. If you do not need to edit the message further, you can give -C option instead.