Flickering screen after simultaneous iOS 18.1 and 26 updates on iPhone 12

ernie ~ Your iPhone 12 could be 5 years old and the long process of applying an iOS update to an iPhone with a weak, aging battery can be the proverbial "straw that broke the camel's back." (And a degraded battery can cause flickering.) So...

Does your iPhone 12 still have its original battery?

And what's the Maximum Capacity % here?...

Settings > Battery > Battery Health & Charging

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Flickering screen is usually hardware. Take it to Apple to be inspected, though if, as we expect, you bought a used phone with a third party battery, Apple will refuse to help.

ernie47-47 wrote: "Bought it last year new. "

ernie ~ Was it bought from a reputable source? ...The steps in the article below may reveal how such an old model 'new' iPhone 12 can have 100% Maximum Capacity:

About iPhone Parts and Service History - Apple Support

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ernie47-47 wrote: "It’s still 100% & original"

That's surprising. Please confirm that you're looking at the Maximum Capacity % and not just the charge %.

Also, did you purchase the iPhone 12 new or used? ...How long ago?

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ernie47-47 wrote:

Please for now would like to know if I should update it back to its previous IOS 18.1 and wait till apple brings out its IOS 26.0.1 or do you have any solutions to this issue..

you cannot revert to ios18, and that battery at 100% cannot be original, or if it is then it is sick.

I would keep it backed up daily and save up for a replacement

You cannot go back to a previous iOS version, unless you are running a beta version. Since you updated to the regular iOS 26, going back to any iOS 18 version is not possible.