How can I update iPhone 6

See instructions in Update your iPhone, iPad, or iPod touch - Apple Support

It can be updated to iOS 12.5.6

alhassan48 Said:

"How can I update iPhone 6: To update my iPhone 6"

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Updating iPhone 6 Plus Past iOS 12:

You cant't. This is not an option for your iPhone model (iPhone 6 Plus). It can only go to iOS10.12. See here: Update your iPhone or iPad Software if you're Experiencing Issues with Location, Date, and Time - Apple Support. So, take this as a good reason to upgrade to a more recent iPhone.

For Future Reference...

Updating an iPhone:

Determining if an Update will Install:

How to do so...

A. Verify Compatibility:

Identify your iPhone model. The first thing that comes to mind in incompatibility being the culprit. So, confirm the model of your iPhone. If iPhone model is incompatible with iOS15 then it is because you iPhone does not meet the hardware requirements for this iOS version(s).

B. If Compatible...

1. Back Up First:

IMPORTANT: If you have an option to update, however, then be certain you backup your iPhone first. That way, you will have something to restore your iPhone from should anything go wrong with the update

then...

2. Proceed with the Update:

Use my User Tip:  Update your iPhone Using a Computer - User Tip