We do get edit requests once a while, where people unknowingly post some sensitive data (e.g. DB user name passwords etc).
If you wish to edit your posts for such a reason, you can send a Purple Moosage to one of the moderators of that particular forum, explaining what you wish to edit and why. If the moderator agrees he might edit it for you. However, you need to realize, the edit is at the moderator's discretion.
There is also very little reason to do so. Posts that old have been cached all over the internet. Editing it here won't edit it there. Once you post something on the net, you should expect it to be there forever.
There are only two hard things in computer science: cache invalidation, naming things, and off-by-one errors