I’m using iPhone 6plus but my iOS is not updating to the new iOS why?

Gideon ~ Your iPhone 6 Plus isn't compatible:

iOS 13 and iPadOS are compatible with these devices

iOS 14 and iPadOS are compatible with these devices

...found by searching Apple's Knowledge Base.

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if you can make do with a smaller screen, you don't need to spend that much. An iPhone SE can be as low as $399. Payment plans may also be available.

https://www.apple.com/iphone/compare/?modelList=iphoneSE2ndgen,iphone6plus,iphone6

iPhone 6 plus does not support iOS 13 and iOS 14 to keep performance good. Seen in the article :https://support.apple.com/guide/iphone/supported-iphone-models-iphe3fa5df43/13.0/ios/13.0

Why have I got to buy a new phone to use my security cameras when I bought my iPhone 6plus I thought it would be my last you have now made it incompatible so you should refund my money very annoying

Because the phone you have is obsolete. It is not capable of running what you want to run.

You have a lot to learn about technology and how it advances.

Not who the "you" is that you think you're addressing here. The iPhone 6 is a 7 years old and does not have the processing power to run an iOS version higher than iOS12.5.2.

Because I’m a a pensioner and have not got a thousand pounds to spend on a new phone

That doesn't alter reality in any way.

Thanks for your reply but I need a phone that will operate on ios14 and it would be to small but thanks anyhow Regards