I want to update my iPhone 6s Plus to iOS 15

Hello,

When updating, your device will update -if it can- to the highest version currently available that it will support. Here are two different ways to ensure the highest iOS that your device will support is installed on your device. iOS 15.7.3 is currently available for an iPhone 6s Plus

For instructions about updating your iPhone, iPad, or iPod touch wirelessly, click here --> Update your iPhone or iPad - Apple Support

Here are instructions about how to update your iPhone, iPad, or iPod touch with the Finder on a Mac computer --> Update your iPhone, iPad, or iPod touch with the Finder - Apple Support