$ echo third change >> myfile
$ hg commit -m 'third change'
$ hg backout --merge -m 'back out second change' 1
reverting myfile
created new head
changeset backs out changeset
merging with changeset
merging myfile
0 files updated, 1 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)