Software Update on iPhone 6S

You say that you cannot afford to pay $1000 USD for a replacement. But you can get a new 128 GB iPhone 16e for $599 USD, or a new 128 GB iPhone 16 for $699 USD. 128 GB is a bit on the small side these days, but the iPhone 6s only came in 16, 32, 64, and 128 GB versions.

Buy iPhone - Apple

There is also the Apple Certified Refurbished Store.

Refurbished iPhone Deals - Apple

The selection there is "catch of the day", and the discounts are usually only about 15% off original price, but there are currently a number of unlocked 128 GB iPhone 14 Plus phones there for $529 USD.

Your iPhone 6s is so old that Apple's "Compare iPhone models" tool doesn't even have an entry for it! But here is Apple's comparison of the iPhone 7, the iPhone 14 Plus, and the iPhone 16.

Apple – Compare iPhone models – iPhone 7 vs. iPhone 14 Plus vs. iPhone 16

Let me count the ways in which a $529 Certified Refurbished 128 GB Phone 14 Plus would be an improvement over your iPhone 6s:

1. The iPhone 14 Plus can run iOS 26. This is the third major update for the iPhone 14 Plus, so if it gets five updates (typical for many iPhones), it would also receive the iOS 27 and 28 updates when they came out.
2. This is because the iPhone 14 Plus has 6 GB of RAM and a 16-core Neural Engine. 6 GB of RAM is not enough to support Apple Intelligence, but it's still triple the amount of RAM that your iPhone 6s has.
3. You would get a bigger screen – 6.7" instead of 4.7". (It's not just bigger, but also higher quality,)
4. The phone would run longer on a battery charge.
5. You would get a higher-resolution "selfie" camera, and on the back, you would get an Ultra-Wide camera. (Your iPhone 6s only has one rear camera and lacks a 0.5x option.)
6. The iPhone 14 Plus supports MagSafe charging.
7. You would gain support for connecting to fast 5G cellular networks.

I don't know whether an iPhone 14 Plus would be right for you, but this example shows that it would be possible to do something short of spending $1000 USD.

The iPhone 6s shipped with iOS 9.0 when Apple first released it more than ten years ago, in September 2015. It received six major OS updates – more than most Android phones get, more even than most iPhones get - but it cannot update past iOS 15.

Apple just released an update to iOS 15 so that iMessages, FaceTime, and activation will work after January 2027, but the iPhone 6s will never get iOS 16, 17, 18, 26, or higher. It lacks the Neural Engine that every iPhone that can run iOS 16 or higher has. It has only 2 GB of RAM, less than any iPhone which can run iOS 17 or later has. Simply put, hardware has advanced quite a bit in ten years; newer versions of iOS rely on the improved hardware, and the iPhone 6s cannot keep up.

If you want or need to run apps that require something more recent than iOS 15, it is time to buy a new iPhone.