Unable to update iOS in my iPhone 6s

What happens when you try, do you receive any message? Make sure your iPhone is at least 50% charged and has a strong WiFi connection. You can view the rest of the requirements to update here by Lawrence Finch, a very knowledgeable member here. This is one of my favorite replies by him.

Keep in mind your iPhone 6s can only be updated to iOS 15.8.3 and no further.

You can view this support article for more information: If your iPhone or iPad won't update - Apple Support

"My phone is not updating"

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Updating an iPhone 6s:

Update Using a Computer:

Have a Computer Handy? If so, try using a computer to update. Go here: Update your iPhone Using a Computer - User Tip. If any updates are availabe, they will install, sure enough.

What happens to you try to update? An iPhone 6s cannot update past iOS 15. What does it say at settings - general - software update?