I have an iPhone 6S ... the thing does not stay charged for long. Do I need to upgrade phone ?

Just needs a new battery. Best Buy or an Apple Store will do it for $50 (it's gone up).

Had mine replaced some months ago and it works like new. I see no reason yet to give up my 6s.

The iPhone 6s is a very old phone now. While it can update to the current iOS version, it would be good to look at a new phone.