How to update iOs15.1

You don't.

An iPhone 6/6 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 15.8.2

• If you have an iPhone 8 or X, you can update to 16.7.9

• If you have an iPhone SE (2nd generation or later), or XR through 15, you can update to 17.6. 1

Which iPhone do you have?

If it's an iPhone 5s, 6, or 6 Plus, you cannot update to iOS 15. For those iPhones, iOS 12 is the end of the line.

If you have a more recent iPhone, you can update to something later than iOS 15.1, but you will not be able to update to iOS 15.1, specifically. Even iPhones for which iOS 15 is the end of the line can update past iOS 15.1.

Update your iPhone or iPad - Apple Support

Updating iOS: common issues and solutions - Apple Community

You can’t

iOS 12 is the last version it can run.

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