How can I upgrade my iPhone 6 Plus to iOS 13.5
How can I upgrade my iPhone 6 Plus to iOS 13.5
iPhone 6 Plus, iOS 12
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How can I upgrade my iPhone 6 Plus to iOS 13.5
iPhone 6 Plus, iOS 12
You can't. Your phone is too old, too underpowered and lacks the hardware required to go any higher than iOS 12.5.6. If you need a higher iOS version for an app, then you need to invest in a newer, more modern iPhone.
An iPhone 6 Plus isn't capable of 13 or above.
(231824)
You cannot.
The hardware is not capable of running anything newer than iOS 12 (the latest version of which is iOS 12.5.6.)
About iOS 12 Updates - Apple Support
How can I upgrade my iPhone 6 Plus to iOS 13.5