Update
hg update [-C] [-d DATE] [[-r] REV]
Update the repository's working directory (the "working copy") to the specified revision of the repository or to the tip revision of the current (named) branch if no revision is specified.
If you forgot to specify -U on "hg clone", doing
hg update null
will remove everything from the working directory of the repository.
Help text: http://www.selenic.com/mercurial/hg.1.html#update