I'm using SVN. Sometimes I miss something when I write a commit message. But once it's been committed, it cannot be reverted, and even I can't edit the message. Why they didn't put the edit function in it?
However, doing this is probably a bad idea. You are, in effect, changing history. One of the points of version control is to maintain a history and audit trail for the project. Allowing arbitrary changes to the history defeats the audit trail. Instead, I would recommend that you perform smaller commits, writing concise yet explicit commit messages, and improving your personal workflow to prevent these errors.
Essentially you have to have admin rights (directly or indirectly) to the repository to do this. You can either configure the repository to allow all users to do this, or you can modify the log message directly on the server.
Check the SVN FAQ here.
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.
The first way is for the repository administrator to enable revision property modifications. This is done by creating a hook called "pre-revprop-change" (see this section in the Subversion book for more details about how to do this). The "pre-revprop-change" hook has access to the old log message before it is changed, so it can preserve it in some way (for example, by sending an email). Once revision property modifications are enabled, you can change a revision's log message by passing the --revprop switch to svn propedit or svn propset, like either one of these:
$svn propedit -r N --revprop svn:log URL $svn propset -r N --revprop svn:log "new log message" URL
where 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.
The second way of changing a log message is to use svnadmin setlog. This must be done by referring to the repository's location on the filesystem. You cannot modify a remote repository using this command.
$ svnadmin setlog REPOS_PATH -r N FILE
where 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.
Because it's a centralized version control system - As soon as you commit a change (and your commit message is by convention bound to the commit), everybody who has read access to the repository can see that information. It is a bad idea to change information after it has been disseminated, because people end up with a different opinion of "reality."
Distributed version control systems like Git alleviate this problem by making sure that the act of making information available to others is atomic and without any additional information like commit messages. But the same principle applies here: You are discouraged from changing things locally which you have already made available to others.