I have an unrooted Samsung ace 2. How do I install cm 12 lollipop on it ?
Your phone is not official supported by CM12, but there is at least an inofficial build some enthusiasts created. Go to CM Unofficial Ports List. There are two inofficial builds created by different teams, so take time and read which one seems better.
But in such inofficial builds not everything may work, also I think you should somehow know what you are doing before starting to flash such ROMs.