Kathy, you can edit typos within 60 minutes from the time of posting a message. Click on the middle button on the right side below the post (I guess it would be a notebook and an eraser, or something like that).
After 60 minutes, you can ask a Moderator to make changes: click on the next button (the triangle with the exclamation). A window will open which will allow you to detail what changes you wish to be made.