I want to know why can’t I update to iso 15.5.7

1.

iOS 12.5.7 is the latest iOS security update for iPhone 5s, 6 & 6 Plus.

Go to Settings -> General -> Software Update -> Download and Install (if you haven't already done that).

iPhone 5s, 6 & 6 Plus can’t be update to a higher iOS (13 and later). 

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2.

If it is an iPhone 6S, 6S Plus, 7 or 7Plus you can update to iOS 15.7.7, you can check by reading this Apple article: iPhone models compatible with iOS 12 -> iOS 16.

iPhone models compatible with iOS 15.5 – Apple Support (UK)

Check first -> iPhone type & iOS version -> Go to Settings -> General -> About.

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

If they are On, turn Off Hotspot & VPN.

Charge the battery > 70%.

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

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

or see:

On which device please?

What is the existing version of iOS?

What happens when you attempt to update?