There's an edit button in all forums. It's a global setting. There is a time limit on editing a post though. I think it's a hour or so. Notice your mistakes quicker.
The reason we do it is to prevent members posting something, other people quoting it, the thread getting a few pages deep, then if a couple of people change their original text, the quoted text isn't right later in the thread, and it can render some long threads to read a bit useless and out of context.
We've also had issues where friday night drinks have caused a guy to have a few responses from regulars, then he's edited his arsey posts saturday, and it's left regulars looking like plonkers arguing to themselves.
Also had an issue year ago where a regular edited all his posts and he changed every single one to some random word. Rendering about 30% of the forum useless.
Just always wise to have edit times set low.