Unable to update my iPhone 6s Plus

What happens when you try??

If your details are correct and you're running iOS 11, then you may need to use a computer to do the update since you've neglected updates for so long.

Go to Settings > General > About

What does it say for iPhone Model and Software Version?

You need an iPhone 6s/6s Plus or newer in order to run iOS 15..

Supported iPhone models - Apple Support

If your iPhone is really an iPhone 6 Plus, then you cannot upgrade beyond iOS 12

If your Software Version is currently 12.5.5, then your phone is not a 6s Plus, it's a 6 Plus

In which case you will need a newer model iPhone in order to run newer iOS versions than 12

have you read thru this support article for updating?

Update your iPhone, iPad, or iPod touch - Apple Support

need more storage for update?

If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support

issues?

Updating iOS: common issues and solutions - Apple Community

video..

Are you sure that you have a 6s Plus, and not a 6 Plus iPhone? The latter cannot be upgraded past iOS 12.5.5.

If you do have a 6s phone, the following could be the reasons that it will not upgrade:

• The specific phone model does not support the update.
• The phone has been "jailbroken."
• The phone is running an iOS beta. If so, then you will need to remove the beta profile first.
• The phone does not have enough free space. Typically it should have at least 5GBs available to update.
• The phone is configured for either or both Low Power Mode or Personal Hotspot.
• The phone is not connected to power and it has less than 50% charge left.
• The phone is not connected via Wi-Fi to the Internet. It won't update on a cellular-only connection … unless it is on certain 5G cellular networks. (The 6 or 6s iPhones do not support 5G.)
• The phone is connected via a VPN.