How to update iPhone 6s iOS 12 to iOS 15.8.1 and iPadOS 15.8.1

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. 

Allen4683 wrote:

Update

See the reply that stedman1 already posted in this thread.

If your phone does not offer you any update, it is very likely that you have reached the end of the line for it. For instance, the iPhone 6 and 6 Plus cannot run iOS 13 or higher. If you have one of those phones and it is running the latest release of iOS 12, no further updates will be available.

Update