my iphone 6 shows 100% charged then it immediately

The solution is to replace the battery. See https://support.apple.com/iphone/repair/service/battery-power

Note that paying to replace the battery in an obsolete device that has not received a major iOS update in over two years is not very cost-effective.

The IPhone 6 is several yeas old and your battery most likely at the end of its life.

Make an appointment at the Genius Bar and have them replace it.