I need to update my iPhone 6 Plus

You likely already have the most recent update for the iPhone 6......iOS 12.5.7

There are no more updates for the iPhone 6 Plus

If you want to run a higher iOS version, you need to have an iPhone 7 to run iOS 15 and an iPhone 8 to run iOS 16, and an iPhone 10 XR or XS or higher to run iOS 17.

To update to the most recent, compatible version of iOS, please see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information.