i am having iphone 6s back camera of 6s is having dust inside.will they give the replacement

Hi

Contact Apple Support (mail-in service may be available), make a Genius Bar reservation or visit an Apple Authorised Service Provider to arrange for your iPhone to be inspected and to establish any available service options (whether under warranty, if eligible, or via chargeable repair service):

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

I'm afraid that no-one here can tell you what Apple will decide (this is a user-based community).

There's also a chat option on the main support page if you'd rather not incur telephone charges.