Unable to update software

iPhone 6 Plus can update to iOS 12.5.5 is the last security updates for iPhone 6 Plus -> released 23 september 2021.

What happens when you go to Settings -> General -> Software Update?

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 need good Service & Wi-Fi.

If they are On, turn Off Hotspot & VPN.

What happens when you attempt to update?

Any error messages?

Which device?

What is the existing version of iOS?