# svn merge -r num_of_rejected_version:num_of_version_to_revert_to pathname

svn merge -r 100:91 ./foo.txt

# commit it
$ svn commit ./foo.txt  -m "Undoing changes from r100; reverting to r91."

http://svnbook.red-bean.com/en/1.0/ch04s04.html#svn-ch-4-sect-4.2

[See also: http://snipplr.com/view/12048]

Oops, didn't mean to commit that?  Undo it easily:

Undoing (reverse merging) changes/versions in Subversion

svn

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