how can i update my iphone 6s?

I successfully updated my iphone 6S to 15.4

Are you sure it is not an iphone 6 or 6 Plus?

Settings, General, About.

If it is 6S then Check

• you are on a good Wi-Fi,

• on charge with more than 50% charged,

• no VPN installed

• low power mode turned off.

• Fully working cellular network connection via your SIM.

• Check Settings, General to make sure you have no Beta profile installed.

Resources:-

Update iphone

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

Iphone will not update

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