Rollback (revert) to a previous subversion revision 

# revert a change to the previous revision
# dry run to see what will happen
svn merge --dry-run -r:revision_to_revert:revision_to_revert_to http://repository-url.com/project/trunk
# do the merge
svn merge -r:revision_to_revert:revision_to_revert_to http://repository-url.com/project/trunk
# and commit it
svn commit -m "reverted from revision_to_revert to revision_to_revert_to"