Why wont my iPhone 6s plus update?

There's multiple steps to installing an update, including verifying the update wasn't corrupted on download and then unpacking it all and installing it. One common problem is a bad Internet connection while verifying.

Frequently a general solution is to try installing the update with a computer.

Click on a link below for full instructions about how to update with wi-fi or 5G (if using 5g then see: Use 5G with your iPhone https://support.apple.com/HT211828 ), or using a computer if you cannot use wi-fi or have storage issues.

Update your iPhone, iPad, or iPod touch. Click here --> https://support.apple.com/HT204204

Or this video:

"How to update your iPhone or iPad" click here --> https://youtu.be/csJAEyvvruY

Update software on iPhone, iPad, or iPod touch in iTunes on PC --> https://support.apple.com/guide/itunes/update-device-software-itns3235/windows

Are you sure you have the correct model information?

- Any iPhone that can't run iOS 15 can't run higher than iOS 12.5.5

- To see a list of 'iOS 15 compatible devices', click here --> https://support.apple.com/HT212712

Here's a simple and fast way to see what is possible with your device, including updating it to the highest sub-version (e.g., 12.5.4 --> 12.5.5) of an iOS/iPadOS you may already have:

On your device, tap:

Settings > General > Software Update.

1. If it says your device is up to date there is nothing more you can do. If apps you are trying to use say you need a higher iOS version, you will need to buy a newer device if you want to run those apps or features.

2. If it says an update is available you will normally only be given one update selection which will be the highest available iOS version supported by your device. However, if you are running an iOS 14 version that is OS 14.5 or greater, you may see two options. Click on a link below for full instructions about how to update wirelessly, or using a computer (see links in document) if you have storage or connectivity issues. If the update listed is not equal to or higher than the iOS version you need to use a feature, you again will need to buy newer equipment if you want to use that feature.

Update your iPhone, iPad, or iPod touch. Click here --> https://support.apple.com/HT204204

Or this video:

"How to update your iPhone or iPad" click here --> https://youtu.be/csJAEyvvruY

You can get better assistance if you provide information leading to a solution such as describing what happens exactly when you try to install it. Are there any error messages, and if so, what are they exactly.

Realize that all we know here is what you tell us, and a surprisingly large number of people do not know basic information about their phones. It helps for us to double check instead of embarking along a long set of trouble-shooting steps based on incorrect information. For example, you said in your original post you have iOS 12 on your phone. That is quite old which means you either haven't updated it in several years, or what I commonly see here, it isn't possible to update it because it isn't the model you think it is.

If you see an error when you update or restore your iPhone, iPad, or iPod, click here --> https://support.apple.com/HT201210

"Learn what to do if you see an error code or alert when you use your computer to update or restore your device."

If your iPhone, iPad or iPod touch won’t update - "You might be unable to update your iPhone, iPad, or iPod touch wirelessly — or over the air — for one of these reasons", with more detail in this link. --> https://support.apple.com/HT201435