You can only edit for aboout 15mins after you post. This was changed a while back, probably a year or more and was because of some trouble somone had caused by posting stuff that people replied to then the original poster edited his posts to make them look foolish. It was then decided that 15mins edit time would be the way forward. In my opinion it was a bad move as the original problem could have been got round by educating people to quote if they thought there would be a needed in future.
You will need to contact an administrator if you wish to edit, cant remember if mods can or not.
Hood