Iphone 7+ Battery draining with IOS 11.2.6

Why is it that with the latest upgrade to ios 11.02.06 that now my battery drains out with in 4-5 hours before it would last 8-10 that's what we were promised with the battery life when we bought it in July 2017. and now it's less than 1/2 the life... ridiculous when i spent $1000 on it less than 1 yr ago!