How can I update my iPhone 6 Plus to iOS 15?

An iPhone 6 plus can't be updated beyond 12.5.7, it doesn't have the hardware required to run anything higher.

If you need a higher iOS version, you will have to purchase a newer phone.

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.7

• If you have an iPhone 6S through 7, or SE 1st generation, you can update to 15.8.3

• If you have an iPhone 8 or X, you can update to 16.7.10

• If you have an iPhone SE (2nd generation or later), or XR through 16, you can update to 18.3.2

You cannot update your iPhone 6 Plus to iOS 15. IOS 12 is the end of the line.

If you do not have money for a new phone, that means that you will need to do without any app that requires iOS 13 or higher.

"How can I update my iPhone 6 Plus to iOS 15?: Please update to ios 15"

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Updating an iPhone 6 Past iOS 12:

A. You can't.  The iPhone 6 will only install to iOS 12.5.7. See here: Update your iPhone or iPad Software if you're Experiencing Issues with Location, Date, and Time - Apple Support. And, for the note, even if you could update, only the latest version will install.

So, what to Try:

B. Update Using a Computer:

Have a Computer Handy? If so, try using a computer to update. Go here: Update your iPhone Using a Computer - User Tip. If any updates are availabe, they will install, sure enough.