How to install iOS 15 on my iPhone 6 Plus?

Justinejaysantos wrote:

How to update iPhone 6

To update to iOS 12.5.7, (it can NOT be updated further) 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. 

The iPhone 6 Plus is not compatible with iOS 13 or higher.

It can be updated to 12.5.7, no further.

Meta has decided to require iOS 15.1 or later for the Facebook and Messenger apps. If you want to run them, you will need to replace your iPhone 6 Plus with a newer iPhone.