How can I update iOS 15.4 on my iPhone 6s

You can update your iPhone to iOS 15.7.4 by going to Settings -> General -> Software Update and installing the particular update.

Make sure your iPhone has

• charged to at least 50%
• preferably connected to the charger
• disabled any VPNs
• disabled Low Power Mode
• connected to a fast, reliable Wi-Fi
• at least 6GB of free storage

However, your iPhone cannot update to iOS 16 due to your internal hardware being too old to support it.

Apple ended major software update support for your iPhone in September 2022 - although it continues to receive security updates - with the most recent being iOS 15.7.4, released just a few days ago.

The iPhone 6S is over 7 years old.

iPhone 8 and later models are capable of supporting iOS 16.

You may consider buying a new iPhone. Even the entry-level iPhone SE can run the latest iOS version, iOS 16.4.

Here is a list of unsupported iPhones with max iOS supported:

Model            Architecture  Introduced       Max supported iOS
iPhone (1st)     32-bit        June 2007        iOS 3.1.3
iPhone 3G        32-bit        July 2008        iOS 4.2.1
iPhone 3GS       32-bit        June 2009        iOS 6.1.6
iPhone 4         32-bit        June 2010        iOS 7.1.2
iPhone 4S        32-bit        October 2011     iOS 9.3.6
iPhone 5         32-bit        September 2012   iOS 10.3.4
iPhone 5C        32-bit        September 2013   iOS 10.3.3
iPhone 5S        64-bit        September 2013   iOS 12.5.7
iPhone 6         64-bit        September 2014   iOS 12.5.7
iPhone 6 Plus    64-bit        September 2014   iOS 12.5.7
iPhone 6S        64-bit        September 2015   iOS 15.7.4 <- Your iPhone model
iPhone 6S Plus   64-bit        September 2015   iOS 15.7.4
iPhone SE (1st)  64-bit        March 2016       iOS 15.7.4
iPhone 7         64-bit        September 2016   iOS 15.7.4
iPhone 7 Plus    64-bit        September 2016   iOS 15.7.4

I did all of these things and yet still wouldn't install. Tried removing install file, restarting and installing, same error. Called Apple support and they didn't have a single recommendation on how to resolve. They were also unable to direct us to a local Apple store. They told us an Apple store wouldn't be able to help me! This is unbelievable! Third time in 2 months Apple support provided no support. This will be the last Apple product my family owns! This company is pathetic.

With the problems that you are having, see if this helps: Updating iOS: common issues and solutions - Apple Community