Best way to switch from 18 to 12 of light?

change the timer on your next off cycle. eg if you are on a 18/6 currently, like lets say 5am till 11pm when it goes past 11pm in its off cycle chnage it to your new 12/12 be it 5am-5pm or 7am-7pm whatever you want. you can do it slowly over the coerce of a week or two like nature does in 15 min intervals a day but the plants don't care. one stop shot, get it done and over with. Good Luck!