If you have a file in subversion (svn) which you’ve updated (and committed) and need to roll back to the previous (or older) revision, you can do:
$ svn merge -rCURR:PREV
(where CURR is the current revision and PREV is the revision to roll back to)
To obtain CURR:
$ svn info FILE
… and to find PREV:
$ svn log FILE
(replacing FILE w/ the file in question)