svnadmin setlog — Set the log-message on a revision.
svnadmin setlog REPOS_PATH -r REVISION FILE
Set the log-message on revision REVISION to the contents of FILE.
You might think that this sounds remarkably like the kind of thing that you can do by using svn propset --revprop to set the svn:log property on a revision. While it does, in fact, accomplish the same end result (changing the log message), setlog does not run any pre or post-commit hooks.
Revision properties are not under version control, so this command will permanently overwrite the previous log message.
--revision (-r) ARG
Set the log message for revision 19 to the contents of the file 'msg':
$ svnadmin setlog /usr/local/svn/repos/ -r 19 msg