How to do iPhone software update?
How do saftwer update
[Re-Titled by Moderator]
iPhone 6s, iOS 11
If your details are correct and you have an iPhone 6s running iOS 11, you are years behind with your updates. If this is true, you will likely need a computer to perform the update.
I would also suspect that you really have an iPhone 6, not a 6s. You can check this by going to Settings > General > About and letting us know what your iPhone model is.
If you really have an iPhone 6, then you cannot update beyond iOS 12.
have you read thru this support article for updating?
Update your iPhone, iPad, or iPod touch - Apple Support
need more storage for update?
If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support
issues?
Updating iOS: common issues and solutions - Apple Community
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