Warranty

It depends on when you purchased your iPhone. The only warranty extension is AppleCare+, which must be purchased within the first 60 days of buying your iPhone. If you have owned your phone longer than 60 days, there is no facility to extend the standard one year warranty further.

Even if the stores are closed, if you are within the 60 day window, you can call Apple directly (in the US) at 1-800-MY-APPLE and ask for AppleCare plan administration.

You can also go to Settings > General > About and if your phone qualifies for AppleCare, the option to add it will be shown there.

What makes you think your battery is damaged? Even if Apple Stores are closed, you can go an Apple Authorized Service Provider.