Where did you buy that iPhone 6, still running iOS 8? If it was brand new, that iPhone was never used and the battery could have suffered a little bit.
But before being worried about this matter: wait for 5-6 days charging the battery overnights and see how it works.
Keep also in mind that I have many friends, iPhone 6 users, that are still using that devices with iOS 11.3 (latest version) and are satisfied.
If the battery doesn't work fine, you can have it replaced at your most convenient Apple Store (there is a worldwide promotion at a reduced price of $29 + taxes in US, but check this price in your country).
Apple - Find Locations - Location Selector
The only strange thing is that it should have automatically installed the 11.3 version (instead of 11.2.6).
But you can update it to the latest version later.
I would also talk to your carrier and ask if they have a software update for your model, running iOS 11.2.6.
See also this guide about automatic updates:
Prevent iOS auto-pushed update.
Regards and best of luck.
Giulio