I want to update iOS 13.0

fizzy ~ Your iPhone 6 isn't listed here:

iOS 13 and iPadOS are compatible with these devices

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

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You can't.

iOS 12.5.5 is the last version for a 6.

It depends that your iPhone is 6 or 6S, can you check the model number?

If it’s 6S, you can update to the latest version, iOS 15.1.1.