Asked but not sloved

You need to install the software offered first .iOS is incremental; get a Wifi connection to do so !

And plug your charger cable in to the mains.

Make sure the phone IS a 6s as the 6 and earlier have been vintaged and no longer get support beyond 12.5.3 other than security patches

Welcome, kashi213, to Apple Support Communities!

If you actually have an iPhone 6s, then you should be seeing the latest iOS 14 release.

If, on the other hand, you actually have an iPhone 6 (not 6s), then iOS 15.5.3 is the latest your phone can use.