I need to update my iPhone 6 Plus

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. 

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.