Why I’m not seeing any update button?

and how do i update my iOS version 12.5.6

The last security updates iOS 12.5.7 -> released September 12, 2022, available for iPhone 6 Plus

Check iPhone model & iOS version -> Go to: Settings -> General -> About.

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

You need about 4 GB device storage and good Service & Wi-Fi.

If they are On, turn Off Hotspot & VPN.

Charge the battery > 70%.

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

You have an iPhone 6 Plus and the highest version that it can be updated to is iOS 12.5.6. You already have the latest version installed on your iPhone 6 Plus. You will need a newer model iPhone to update to IOS version 15/16.

Axel F.