Can I use a different battery inside my iPhone 6s to make it last longer?

iPhone batteries degrade after the first 6 months or so.
If your iPhone battery is behaving the way it initially did, you can get a battery case. If it’s degraded, you can replace the internal battery.

Apple has made the battery very easy to replace. You can replace it on your own for about $30. That includes the price of tools, adhesive, and a battery.
Here’s a step-by-step guide to replacing a battery on your iPhone.

• Unscrew the 2 pentagonal screws from the bottom of the phone on each side of the lightning port.
• Use a heat gun to heat the display of the iPhone to a temperature that is just too hot to touch.
• Wedge a prying tool or a thin guitar pick into the seam between the glass and metal.
• Slowly pry the glass up. Do not touch the top part of the phone. Be careful not to slice any ribbon cables. They will snap even if you look at them the wrong way.
• Now that the glass has been pried off, disconnect the cable connecting the logic board to the battery. Be gentle.
• Pull the two white tabs under the battery. If they snap off, you will have to use a plastic prying tool to pry the battery off of the metal back. Do not use a metal prying tool for this. Doing so will turn your iPhone into a Note 7. Do not puncture the battery.
• After the battery is out of the phone, get your new battery and remove the white paper from the adhesive. If your battery doesn’t come with adhesive, use thin double sided tape.
• Put the new battery in the phone. Make sure to get the right orientation.
• Connect the fragile ribbon cable.
• Use a new adhesive strip to put the screen back in place.
• Screw the two screws back on.
• If all done correctly, you have successfully replaced your iPhone’s battery.

If you are scared of doing something wrong, you can chicken out by going to an unauthorized repair center. Yes, that’s right, an unauthorized repair center.
Replacing batteries is really cheap and Apple will ask for much more money than you have to shell out.
Pick out a decent looking repair center, and ask them to fix it for you. If they ask for more than $50, you should go elsewhere.