I would top it just above the lowest two fan leaves and then get it under more light, like has already been said. As long as you leave at least two fan leaves, your plant should recover nicely from the topping and will end up much more bushey than it is now. I top all of my plants right after they go into veg and it makes them into little bushes. It does add some time to your veg, but it's worth it in the long run IMO.