Charging batter correctly

Hi , Im just confused how charging my iphone 6 correctly , i searched a lot about that , many says= that charging it at night doesn't hurt or charging to 100% also does not have an effect but others say that charging it at 80 or 85% is the best ! and pleas how to know that i charge my iphone in 4.20 v does it mean charging it slowly or fast ? i'm just don't know how to charge it properly ! ( my iphone drops every 5 or 6 minutes 1% is it fine or i should do a reset )