Can I upgrade iPhone 6s Plus to iOS 15?

iOS 15 is compatible with the iPhone 6s Plus so you should definitely be able to update. If you're having trouble updating try the instructions detailed here: Update your iPhone or iPad - Apple Support

The full list of iOS 15 compatible devices is here: iPhone models compatible with iOS 15.5 – Apple Support (AU)

According to your profile, you have an iPhone 6s Plus and that model should be able to update to iOS 15.7.1

What error message do you see on the screen when you attempt to update your iPhone 6s Plus to iOS 15.7.1?

Axel F.

annmarie266 wrote: "Yes updated successfully what advantages does it have"

annmarie ~ Read this:

About iOS 15 Updates - Apple Support

...Found by searching Apple's Knowledge Base.

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Yes updated successfully what advantages does it have

The 6S/6Splus can upgrade to iOS 15 not to iOS 16.

The 6/6plus cannot upgrade further then iOS 12.5.6 due to hardware limitations