$ \textbf{echo third change >> myfile}
$ \textbf{hg commit -m 'third change'}
$ \textbf{hg backout -m 'back out second change' 1}
reverting myfile
created new head
changeset  backs out changeset 
the backout changeset is a new head - do not forget to merge
(use "backout --merge" if you want to auto-merge)