How much $ to buy new iPhone 6 battery and put old one in another iPhone 6 and new one in this one

Hi dirbohme,

Thanks for reaching out! You can find information about iPhone battery replacement costs in this Apple Support article: iPhone Battery Replacement - Official Apple Support (CA)

If you would like to go ahead with the Battery Replacement, you should reach out to Apple Support and book an appointment!

Cost for a battery change -at authorizes centre ie London drugs, 65$ after tax, but more if you want cleaning out the ports

Both batteries need to be replaced. A battery that has 82% capacity is close to the end of its life.