I want to update my iPhone 6s to iOS 15

Assuming you have a compatible device, see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

Ajmitchell69 wrote:

i want to upgrade my iOS to next version 15 upward

And to answer your question, an iPhone 6s tops out at iOS 15, and cannot be updated further.

Apple requires iPhone 8 or newer for iOS 16, and does not support iOS 16 on iPhone 7 and earlier.

For any requirements including iOS 16, you will need a newer (used, refurb) or new iPhone.

Any recent iPhone will be substantially more capable than an iPhone 6s, as well.

To update your iPhone 6s to iOS 15, remove any beta profile, remove any add-on VPN client, force-restart the iPhone, and try the update again. If that fails, use a Mac or Windows PC to update.

… How to uninstall iOS or iPadOS beta software - Apple Support

… Force restart iPhone - Apple Support

… Update your iPhone or iPad - Apple Support (includes how to use a computer)

You may not realize it but you are not posting to Apple Chat with live agents. You are posting to a community forum where Apple users try to help other Apple users. See the word Community at the top, left.

Ajmitchell69 wrote:

why no response

Because it sometimes will take the volunteers here more than 30 seconds to respond.

..and some of us haven't fast enough internet to reply before a day goes by..😳