I Phone 6 S battery is draining fast .

Can you please give us some more details?

How fast is it draining?

When did you start noticing this?

Settings - General - Battery - Battery usage - which apps are listed as having highest use?

Also try a forced restart as a process may be "stuck"

Restart your iPhone, iPad, or iPod touch - Apple Support

Derek from Malaysia wrote:

I have been checked the highest use is clash of clan ...what should I do ? I already opened low power mode .I got open cellular LTE ,but if I just locked the phone screen why the battery percentage still draining...

There could be some process ongoing despite the locked screen

I would test this

Force restart the phone like it says in the article above

Shut off all the open apps using app switcher

Then charge the phone to 100%

Put it on airplane mode for about an hour and see how much it drains

Please let us know