IPhone 6S battery problem

I have the same iphone as you and i lost all my battery when im in the winter and if you still have you should buy and new iphone or repare it.

You should try getting a battery replacement. It depends on your current battery status, if It's bad or not. Goto battery > Battery health and tell me the results.

Yeah you need to upgrade your phone apple let out a code to make older versions of iPhones slow down.

Its actually quite easy to fix an iPhone battery. There are great guides on fixit and you can order proper toolkits there too. Although Apple does its best to discourage home repairs, its really easy with the help of the internet

I would buy a bunch of power banks and charge it when I need to. Walk around with them!

buying a new battery is easy what u could do is order it online and get the teardown manual from ifixit

It is simple to obtain a new battery. You could order it online.
<a href="https://support.apple.com/en-us/HT208387">Try it.</a>