Battery

Nothing you can really do about it other than pay for a replacement if you feel you want a battery with more capacity. Apple will only replace a battery under warranty if there's a specific battery fault. Something like a bad battery resulting in an unplanned shutdown or if it actually reaches their standards for "depletion" (under 80% battery health) before it reaches 500 complete charge-discharge cycles within the warranty period. Everything in between is normal variation based on way too many variables for anyone to account for.

If you want to be able to use it longer, maybe pack a USB power bank with you. I've got a Mophie Powerstation 8000 that fits nicely in my pocket. It can extend my iPhone runtime by about 20-24 hours.

If it is still operating properly do nothing and wait for it to hit 80% and have it replaced. Otherwise click on the Get Support link at the top right of this page to contact Apple Support and they can test your phone/battery remotely.