Unable to update my iPhone 6s from iOS 13.7 to 15.8.3

"Unable to update my iPhone 6s from iOS 13.7 to 15.8.3: my phone is Apple iPhone 6s Plus, Capacity = 128GB, its software version is 13.7 and can't update to 15.83. What can I do to resolve this issue? "

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Updating an iPhone:

For your model (iPhone 6s), you can also try updating using a computer. The benefit, is that you can connect through Ethernet, if WiFi is giving you issues. See my User Tip: Update your iPhone Using a Computer - User Tip

Back Up First:

IMPORTANT: If you have an option to update, however, then be certain you backup your iPhone first. That way, you will have something to restore your iPhone from should anything go wrong with the update

then...

2. Proceed with the Update:

Go Here:  Update your iPhone Using a Computer - User Tip

I've seen threads on these forums saying that sometimes, if you skip enough iOS updates, you need to use a Mac or PC to update your iPhone.

You may also want to look at this User Tip:

Updating iOS: common issues and solutions - Apple Community

See this article.

If your iPhone or iPad won't update - Apple Support

We'll need more information before we can provide any useful suggestions. What exactly happens when you try to update? Error messages? And how are you trying to get the update, cellular data or Wi-Fi?

Regards.