Battery Health cannot be restored. Battery Health goes one direction only and that is down. A decline of 1-2% per month is considered within normal range and since you're owned your phone as you said for 9 months, your decline is not unexpected or unusual. Batteries are consumable parts and with every full cycle battery charge a battery loses a little bit of its health, when compared to when it was new. Its what batteries do. ALL BATTERIES.
When the health of your battery dips to 80% or less, which it will do, it will then be time to pay Apple to put a new battery in your iPhone. Until then, make sure you don't let the battery discharge below 20% and NEVER to 0%, and make sure you plug your iPhone in to charge when you go to bed at night, with Optimized Battery Charge turned on in Settings and leave it on charge all night EVERY night.