How to update

The iPhone 6s Plus can be updated to the latest release of iOS 10.15.

You may update your iPhone in the Finder on a Mac running macOS 10.15 Catalina or later.

Please follow that guidance here: Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

OR

You can update your iPhone wirelessly without a computer.

You will find guidance for that here: Update your iPhone or iPad - Apple Support

Jungido Said:

"How to update"

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Updating an iPhone 6s Plus:

Update Using a Computer:

Have a Computer Handy? If so, try using a computer to update. Go here: Update your iPhone Using a Computer - User Tip. If any updates are availabe, they will install, sure enough.