Topping a cannabis plant is unnecessary, you know (I hope). If you're looking for cutting fodder for rooting clones, snip a bottom branch. If merely wanting to encourage branching, you only need to train the plant, in other words, pull that top down and tie it in place. If you're worried that you might break the main stem (having to "splint and tape" it back together is annoying, I know - although everyone should have the opportunity to experience it... once
), just don't get into a hurry. You can train actual trees into all kinds of contortions, lol; cannabis is pretty easy. Little bit today, little bit more tomorrow, more still the day after... And you get to keep the top of your plant.