HOW to switch to 12/12

I am wondering how to switch lighting from 18/6 to 12/12...By how I mean which of the light hours do I turn into dark period hours.

Right now the light schedule is:

Lights on: 8PM

Lights off : 2PM

SO.....Do i make lights off earlier at 8AM?
OR.....Do i make lights on later at 2AM?

since the lights go off at 2pm turn back on at 2am