svnadmin setlog — Set the log-message on a revision.
Set the log-message on revision REVISION to the contents of FILE.
This is similar to using svn propset --revprop to set the svn:log property on a revision, except that you can also use the option --bypass-hooks to avoid running any pre- or post-commit hooks, which is useful if the modification of revision properties has not been enabled in the pre-revprop-change hook.
Revision properties are not under version control, so this command will permanently overwrite the previous log message.