How can I update my iOS 12.4 to 15.4
Software update
iPhone 6s Plus, iOS 12
Software update
iPhone 6s Plus, iOS 12
If you truly have an iPhone 6S +:
Update your iPhone or iPad - Apple Support
If you actually have an iPhone 6+, the latest update it can use is iOS 12.5.6.
If you truly have an iPhone 6S +:
Update your iPhone or iPad - Apple Support
If you actually have an iPhone 6+, the latest update it can use is iOS 12.5.6.
What does it say at settings - general - software update?
How can I update my iOS 12.4 to 15.4