For this tutorial step, we have prepared a repository with some ready made commits to speed things up.
You can download the repository here.
In this step, we are going to undo the previous two commits using the reset command.
Go to the downloaded "stepup-tutorial/tutorial3" directory and examine the history of this repository. It should look like the following.

Let's confirm the branch's history using the log command.
$ git log
commit 0d4a808c26908cd5fe4b6294a00150342d1a58be
Author: yourname <yourname@yourmail.com>
Date: Mon Jul 16 23:19:26 2012 +0900
append description of the pull command
commit 9a54fd4dd22dbe22dd966581bc78e83f16cee1d7
Author: yourname <yourname@yourmail.com>
Date: Mon Jul 16 23:19:01 2012 +0900
append description of the commit command
commit 326fc9f70d022afdd31b0072dbbae003783d77ed
Author: yourname <yourname@yourmail.com>
Date: Mon Jul 16 23:17:56 2012 +0900
append description of the add command
commit 48eec1ddf73a7fb508ef664efd6b3d873631742f
Author: yourname <yourname@yourmail.com>
Date: Mon Jul 16 23:16:14 2012 +0900
first commit
Open sample.txt and verify that the text content looks something like below.
Git commands even a monkey can understand
add: Register a change in an index
commit: Save the status of an index
pull: Obtain the content of the remote repository
Use the reset command to delete the previous two commits like below.

$ git reset --hard HEAD~~
HEAD is now at 326fc9f append description of the add command
If done correctly, sample.txt should no longer contain the last two lines "commit: Save the status of an index" and "pull: Obtain the content of the remote repository". You can also verify that these commits are no longer in the history with the log command.
$ git log
commit 326fc9f70d022afdd31b0072dbbae003783d77ed
Author: yourname <yourname@yourmail.com>
Date: Mon Jul 16 23:17:56 2012 +0900
append description of the add command
commit 48eec1ddf73a7fb508ef664efd6b3d873631742f
Author: yourname <yourname@yourmail.com>
Date: Mon Jul 16 23:16:14 2012 +0900
first commit
ORIG_HEAD points to the original commit before reset actually takes place. This may come in handy especially when you accidentally issue a reset. You can restore the previous history by executing a reset to ORIG_HEAD.
$ git reset --hard ORIG_HEAD
HEAD is now at 0d4a808 append description of the pull command