Why is my iPhone 12 not updating?

Make sure you are abiding by all the condtions for the method you aere trying to use:

Update your iPhone or iPad - Apple Support - wirelessly

Update iOS on iPhone - Apple Support - with a computer

If your Apple mobile device won't update, read this support article for a list of things to try --> If your iPhone or iPad won't update - Apple Support

Another, less common reason for not being able to update is if you were using a beta version of software. Refer to How to uninstall iOS or iPadOS beta software - Apple Support

iPhone 12, iOS 14

A question: 

1.

Do you have a MDM profile?

If so, contact your company - IT department.

2.

Have you install / use iOS Beta version?, if so remove the beta profile (Settings -> General -> VPN & Device Management).

Then Restart your iPhone and check for updates again.

If neither applies, see ⤵️

You can update your iPhone to iOS 16.4.1 -> iPhone models compatible with iOS 16 – Apple Support (UK)

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

What error message appears when you try to update?

I would really help if you could provide a screenshot.