Can you replace an iPhone 6s back cover

no

Hi

Contact Apple Support (mail-in service may be available) or make a Genius Bar reservation to arrange for your iPhone to be inspected and to establish any available service options, likely timescales and cost:

• Contact - Official Apple Support
• https://twitter.com/applesupport
• http://www.apple.com/retail/geniusbar/

Apple has never done purely cosmetic repairs. There also is no "back" to replace. The aluminum iPhone case is a single machined unit. The only way to replace the case would be to replace the entire device, but Apple only offers that if there is some actual malfunction with the device, not merely cosmetic blemishes.