Log messages are kept in the repository as properties attached to each revision. By default, the log message property (svn:log) cannot be edited once it is committed. That is because changes to revision properties (of which svn:log is one) cause the property's previous value to be permanently discarded, and Subversion tries to prevent you from doing this accidentally. However, there are a couple of ways to get Subversion to change a revision property.
$ svn propedit -r N --revprop svn:log URL $ svn propset -r N --revprop svn:log "new log message" URLWhere N is the revision number whose log message you wish to change, and URL is the location of the repository. If you run this command from within a working copy, you can leave off the URL.
$ svnadmin setlog REPOS_PATH -r N FILEWhere REPOS_PATH is the repository location, N is the revision number whose log message you wish to change, and FILE is a file containing the new log message. If the "pre-revprop-change" hook is not in place (or you want to bypass the hook script for some reason), you can also use the --bypass-hooks option. However, if you decide to use this option, be very careful. You may be bypassing such things as email notifications of the change, or backup systems that keep track of revision properties.