Let's do a quick test to see if the battery is easily at fault.
1. Plug the phone to a charger.
2. When you see apple logo immediately unplug the charger.
3. Did the apple logo off immediately from unplugging the charger?
- If yes, the battery is either not plugged in or it is faulty and needs to be replaced.
- If no, Your battery is fine.
If you would like to replace the battery yourself I have provided guide and link to the replacement battery kit:
iPhone 6s Battery Replacement
iPhone 6s Replacement Battery
Note that apple is currently doing discounted battery replacements so whatever is more convenient for you, though Apple gets bombarded with battery replacements so there is a long queue for that there.
Taking it to a mobile phone shop is another alternative too if you need it done on the spot.