How do I update my iPhone 6s Plus to iOS 16?

Hi,

iphone 6S plus will sadly not get Ios16 ever (see photo below) so unless you use the mail app from apple or buy a new iPhone you cant use gmail anymore in app

Note that it may be possible to connect your Gmail account to the Apple Mail app, in which case you wouldn't need the Google Gmail app to read your e-mail on your phone.

Add an email account to your iPhone, iPad, or iPod touch - Apple Support

I need up date my Truist bank app to iOS 16

Apple introduced the first iPhone 6s Plus models in September 2015 – about nine and a half years ago. At that time, they were running iOS 9.0. The iPhone 6s Plus received six major iOS updates, and can run the latest release of iOS 15 (currently iOS 15.8.4) – but iOS 15 is the end of the line.

You cannot update an iPhone 6s Plus to iOS 16 or higher. If you need or want a higher version of iOS, you will need to purchase a new iPhone.