Topping an auto strain?? newb to auto-strains.

hey guys, got an auto ak 4 weeks in under leds it is about 6 inches tall and quite bushy. A bit like like Gioganja's 2nd photo. Never grown before. They have been under 18/6 for 4 weeks. When do I switch to 12/12? My mates are telling me to top them. I have no idea how and I dont wanna risk f~ckin up the small yield I will be getting. Obviously under my setup they wont be max yield but can I expect plants to grow more during flowering?? any input would be appreciated

dude ive grown a few autos before i had to move my crop outdoors due to security issues dont top most strains react great to 20/4 light schedule they seem to yield a bit more rather than 18/6 it makes no sense to go 12/12 since the strain flowers on life span night light schedule like sativa or indica