How do you edit previous postings???

You can edit something you previously posted - but only for about 30 minutes after you first post it. (It may be 60 minutes - the point is its a short amount of time, you can't edit something you posted hours or days ago.) If you're still within the allowed amount of time, there's an Edit button in the post - I forget exactly where - but it's as obvious as the "quote" and "quick reply" buttons are. Hope that helps!