MY short answer is : don't. If you want to try your hand at topping, grow a photoperiod... autoflowers have their own schedule built in to them and you never know exactly what that is.... NORMALLY, autos go into pre-flower around Week 4 - but I've had some pre-flower in Week 3 and some in Week 5... Topping is a very stressful event for the plant and it may not have time to recover. Have some done it successfully? Yes, of course... but is it WISE? No... Do some good LST on her instead to open up the lower bud sites and make them as strong and as big as you can...
Good luck...