The normal way is to hit the esc key to undo changes (or, use the menu and
go edit->undo).

If you **really** want to place a button, then you can use the following
code. However, it likely better to teach/train the users to use the ESC key,
or the edit->undo, since that will work for ALL forms. Otherwise, for
virtually for EVERY form and application you make, you have to add a button
called:

Exit without Save.

and, if they use an access application written by someone else, they not
have that button, and once again, you would have been better off to teach
the users about edit->undo....

Anyway, here is the code that you can place behind a button that will Exit
without save
(thus, it will work for existing records, and new ones ).

  If IsNull(Me!ID) = False Then
     If Me.Dirty = True Then
        Me.Undo
     End If
  End If

  DoCmd.Close

note: replace !ID with the name of the primary key used for the the forms
recordsource (table)

Lets just hope users don't start hitting that button...and you be back here
in a week asking why so many users are complaining that the record did
not get saved/added...

1. Set the command button's On Click property to:
   [Event Procedure]

2. Click the Build button (...) beside this.
Access opens the code window.

3. Set up the code like this:
   Private Sub Command1_Click()
       If Me.Dirty Then Me.Undo
       DoCmd.Close acForm, Me.Name
   End Sub

That will work unless Access has already saved the record before you click
the button. There are many things that could cause this to happen, e.g.
tabbing through the last control on the form (so it moves to the next
record.)