Updating sofeware
Why i cannot update my iphone 6 plus
iPhone 6 Plus, iOS 12
If you're trying to update to 13 or above, because that model doesn't have enough RAM.
If you're trying to update to 12.5.4, it depends on what happens when you try.
If you're trying to update to 12.5.3 or earlier, because Apple no longer signs it.
(200948)