How can I update my phone to iOS 15.6.1

Go to Settings -> General -> Software Update -> Download and Install.

You need about 6 GB device storage and good Service & Wi-Fi.

If they are On, turn Off Hotspot & VPN.

If it is an iPhone 6S you can update the iPhone to iOS 15.6.1, see this Apple article:

iPhone models compatible with iOS 15.6.1. -> iPhone models compatible with iOS 15.4 – Apple Support (UK)

See the following Apple articles:

Update your iPhone, iPad, or iPod touch

Update your iPhone, iPad, or iPod touch - Apple Support (IN)

If your iPhone, iPad or iPod touch won’t update

If your iPhone, iPad, or iPod touch won't update - Apple Support

You can do the update via PC, see this Apple article: Update software on iPhone, iPad or iPod touch in iTunes on PC

Update software on iPhone, iPad or iPod touch in iTunes on PC – Apple Support (UK)

If you have a Mac see this Apple article: Update your iPhone, iPad, or iPod touch with the Finder

Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

Go to Settings > General > About

What does it say for iPhone Model and Software Version?

You need an iPhone 6s/6s Plus or newer in order to run iOS 15..

Supported iPhone models - Apple Support

If your iPhone is really an iPhone 6, then you cannot upgrade beyond iOS 12 (currently 12.5.6)

have you read thru this support article for updating?

Update your iPhone, iPad, or iPod touch - Apple Support

if your phone is compatible but you're still on an older iOS, then you may want to try using a computer to perform the update

need more storage for update?

If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support

issues?

Updating iOS: common issues and solutions - Apple Community

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