Can I update a 12.5 ois operating system on my iphone 6 to 14.0?

You might be able to get a compatible version by downloading the current one to a newer unit on the same Apple ID first; otherwise, you'll need to buy an iPhone 6s or other more-recent model, but not a brand new one.

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No. The iPhone 6 cannot run anything higher than 12.5.5

My mobile name iPhone 6 Plus.My mobile is not updated

The 6 Plus cannot run anything higher than 12.5.5.