Best not to top autoflowers, their recovery time will prevent the plant from growing very big before the flowering commences.
If you are interested in controlling height, a bit of tying down or "low stress training" as people call it, will ensure more growth without any set backs, which will then lead to a bigger harvest. Topping will involve your plant taking 7-14 days to recover and continue growing properly, by which time flowering may commence and this will lead to an undersized plant flowering, with minimal size, and with minimal yield/harvest.
Hope this helps,............
Organoman.