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.
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.
--revision (-r) ARG --bypass-hooks
Set the log message for revision 19 to the contents of the file msg:
$ svnadmin setlog /usr/local/svn/repos/ -r 19 msg