can't upgrade

Yes, the iPhone 6s can be updated to iOS version 15.6.x. What are the error messages that are displayed on the iPhone when you attempt to update it?

Make sure that your iPhone is setup to meet the following requirements before you attempt to update the iOS version over a Wi-Fi Network. Open Settings ➡️ General ➡️ Software Update: If there is an update available for your iPhone then it will be listed here. Follow the on-screen instructions to download and update your iPhone. 

• Your iPhone must have a fast Wi-Fi connection; you cannot update over cellular data or a cellular hotspot
• Ensure that you do not have a beta profile installed from a previous version
• The iPhone can not be in Low Power mode
• The battery state of charge must be above 50%
• Your iPhone must have at least 6GB or more free storage space
• Your iPhone cannot be connected through VPN
• You should leave the phone plugged in until it finishes updating, both to assure that it remains above 50% and to keep Wi-Fi enabled (Wi-Fi turns off when the phone sleeps if not connected to power).

Axel F.

Dannymac22 Said:

"can't upgrade: My wife has a 6S with 13GB free and absolutely can't get the iOS to upgrade to 15.6 (from 14.3). My understanding is that the 6S ought to be good with 15.6. Yes? If so,what can she do?"

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Updating an iPhone 6s:

Do so using WiFi or Ethernet on a computer.

Use 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.

In addition to cycling power, try a Forced Restart on the phone.

Force restart iPhone - Apple SupportOfficial Apple Support › guide › iphone › ios

It would not hurt to try updating using a computer if all else fails.

It says iOS 15.6 is available? or iOS 15.6.1?

If it says 15.6, then that's not right..

Does it show that it's downloaded? If 15.6 was downloaded and sat there for a month without installing.. could be a problem.

Check Settings > General > iPhone Storage and see if an iOS update is in the list. If so, delete it. Restart the phone. Check again.

Otherwise, the phone is 9 months behind with updates, might need to use a computer to do the update.

I mean.. you should be able to go directly to iOS 15.6.1 from iOS 14. Often times people that are way behind in updates, especially when it's a different iOS.. can have some trouble updating, so it's just a suggestion that may resolve the issue.