A lot of my family are Jehovah's Witnesses and they have a lot of similarities to Mormons. From what I've seen the best way to leave the church is just stop attending the meetings. Unfortunately, there's no easy way to tell your parents you're no longer a believer, you just have to do it. Most likely they will know anyway, or at least suspect it.
Oh good luck with that one ! Xx
Just tell them. They'll get over it eventually.