can i get a replacement battery for my iphone 6S PLUS, and how hard is it to replace

If you "know nothing," iPhone Battery Replacement - Official Apple Support provides replacement options.

Apple does not sell or make parts available to individuals.

Apple policy is there are no user serviceable parts in any iPhone.

Contact Apple Support, I believe a new battery costs about $50 if you go through Apple.

And that will not be an Apple battery that you had installed and Apple most likely will not provided any future service to the device. Also don't put any reliance on your iPhone have any water resistance. Have you tried to check your Battery Health?

Don't do it Shirley, take it to Apple and pay your $49.

Hi, you can't replace the battery on your phone by yourself.

You can take your phone to an Apple Store or Apple Authorized Service provider to get the battery replaced. You can book an appointment on getsupport.apple.com or use the Apple Support app.

We were able to get one insttted in 20 minutes by one of those phone booths