Cant install 15.1 for my iPhone 6s.what is the reason?help me

Are you sure you have a 6S and not a 6?

Go to Settings > General > About and look at your version.

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.5

• If you have an iPhone 6S or newer, you can update to 14.8.1 or 15.1

Go to Settings > General > Software Update.

Update your iPhone, iPad, or iPod touch -

https://support.apple.com/en-us/HT204204

Can you post a screenshot, it could reveal why you aren't able to update.

Make sure you:

• Are using a WiFi connection (not a hotspot).

• Have your battery charged at least 50%. Better yet, keep it plugged in during the update.

• Are not using a VPN connection.

• Do not have power save mode turned on.

• Do not have a beta version installed.

• You have at least 6 gigs of storage available.

You can also check out this article for help -

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

https://support.apple.com/en-gb/HT201435

Asankasanjeewa78 Said:

"Cant install 15.1 for my iPhone 6s.what is the reason?help me"

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What the Issue?: Perhaps there is an error connecting to Apple's servers?

To Check for Updates:

Use a Computer and Ethernet:

Start with attempting an update through use of a computer. If you cannot use Wi-Fi, then use Ethernet.  Go Here: Update your iPhone Using a Computer - User Tip. On the computer, you would connect it through Ethernet, going directly into your router. For a Mac (or PC) that does not have Ethernet, this would be the adapter to use: Belkin USB-C to Gigabit Ethernet Adapter - Apple. If you get the same error, the model is likely just not compatible.