Edit a post/topic?

i have edited one of my forums before but i can´t find the pencil anymore which is needed to edit a topic/post i have made.

in short my question is how do i get my posts edited?

also i´m looking to this topic now and the pencil sign is just here now, but it dissapeared with my other forums? why is this???

does anyone know why this is and what i can do about it?