The battery of my iPhone 6s think drains faster than normal. I need help.

First, force restart the phone by holding the sleep and home button for about 10sec, until the Apple logo comes back again. You will not lose any data by doing that, but it can cure some glitches.

If this does not work, and to make sure that this is not software related, set it back to factory settings, without using any backup data afterwards. Set up the rest of the personal settings manually and test the phone.

If the battery works again as usual, restore from the latest backup. If the problem comes back, the backup data contained corrupt data, causing your issues.

If it still does not work, this is a hardware issue and the phone has to be serviced by Apple or an Apple Authorized Service Provider:

Use iTunes to restore your iOS device to factory settings - Apple Support

Find an Apple Authorized Service Provider

iPhone - Contact Support - Apple Support

An iPhone can only be serviced in the country of purchase. Apple USA will only check your phone if it has been bought in the USA.