how do I remove the battery on my Iphone 6S?

Better go for --> iPhone Battery Replacement - Official Apple Support

That operation must be done by Apple Specialists at an Apple Store or Authorized Service Provider, keeping in mind that Apple doesn't sell nor distribute any genuine/original spare part to anyone, except to his official stores.

If you still want to replace your battery by yourself, do it AT YOUR OWN RISK, finding instructions in Internet.

For your iPhone model, in the US, the operation (tax escluded, work included) costs $49.

If you're out of USA, check that price in your country.

Best of luck.

Giulio