iPhone 6s won't hold a charge

Best Buy is an authorized Apple Service shop. 86% would normally indicate that the battery was replaced about 12-14 months ago.

Take the phone back to Best Buy and ask them to run a check on the battery. If you have an Apple Store in your area, you can take the phone there as well, but make an appointment before you go.

What is the Battery Health / Maximum Capacity?

Settings > Battery > Battery Health and Charging > Maximum Capacity

Hello ~ Where did you have your battery replaced? Apple?

~Katana-San~

Hello again ~ I would take it back especially since it is at 86%. When you say fairly new…how long do you think since it was replaced?

~Katana-San~

Hello ~ That may be about right. Take it back in for a check and/or another battery.

~Katana-San~