If you are planning on topping you should wait at least two or three more weeks before you do. Your plant, to me, looks much too young to be topped. You could end up doing much more harm than good. You want to wait till the plant is a little taller and more developed before you do the procedure.
When you top you are making the plant revert the majority of its growth hormones to the first two off branches, instead of the tip.
With this being said when you top you want to remove the very tip-top "shoot" or cluster of leaves. I use a razor and slice off the top just above the node at the top, the part of the plant that new leaves are "sprouting" out of.
This allows all the energy to go to the first two off branches instead of the top, allowing two main colas instead of one.
Now, with this said,
If you are going for a quality over quantity grow and can wait an extra few weeks than topping is recommended.
I have grown many crops that I have topped, trimmed, trained, stressed and bent. I have found that stressing the plants in these manners will produce slightly more potent and sometimes more tasty bud.
Sometimes, this is not in fact a rock solid claim due to variations in the outcome.
But if you are planning on progressing in the CannaArts than why not give it a try once or twice and see what happens?
Thats what I did anyways...