Dude you've got bigger problems that topping isn't going to solve. They are stretching from lack of light possibly, and it looks like you're having issues with deficiencies or bad media.
Topping could stress the plants out more. It is best to top a healthy plant. You just cut the top off 5 or 6 nodes up or where ever you want. Cut just above the node, where the two lateral shoots there are growing in (they'll take over as the leaders).