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To keep the changes from the commit you want to undo

$ git reset --soft HEAD^

To destroy the changes from the commit you want to undo

$ git reset --hard HEAD^

You can also say

$ git reset --soft HEAD~2

to go back 2 commits.

If you are on Windows you will need to put HEAD or commit hash in quotes.

$ git reset --soft "HEAD^"
$ git reset --soft "asdf"

Source:
http://stackoverflow.com/a/13480388/2050561