Although generally, you don't want to edit an archive (the whole point is that it records what actually got sent out to your list members!) there is the odd time where it may be necessary. People accidentally send private information to a publicly archived list, you want to remove spam links, etc.

If you want to remove the post entirely, see How can I remove a post from the list archive or remove an entire archive?

If you want to edit the post, there are two options. Both of these require shell access to your mailing list's files. If you do not have this access, please ask your system administrator to do this for you:

1. The easy way to do it is just to edit the HTML that's displayed. That can be found in archives/private/ in a directory that shares the listname of your list. Inside that directory there will be a bunch of different subdirectories, probably per-month. (eg 2008-December, 2009-January). You'll have to go find the problematic message or messages in there and edit them as HTML files.
2. The "right" way to do it is to edit the list's mbox file (found in archives/private/LISTNAME.mbox/LISTNAME.mbox – yes, that last bit is repeated: the first is the directory, the second is a text file) and then regenerate the archives using bin/arch, but if you go this route, you'll need to shut down mailman while you're editing the file, otherwise if someone sends a message to the list while you're editing, it won't get archived. The benefit to this method is if your archive ever has to be regenerated then no one will need to do the editing again, but honestly, most people never regenerate their archives so it's probably easier and safer to use option #1.

Related FAQ entries: How can I remove a post from the list archive or remove an entire archive?

This is one of many Frequently Asked Questions.