How do I manually update my iPhone 6 to ios14 or 15

The iPhone 6 is not compatible with iOS 13 or higher. The highest possible for that model is iOS 12.5.5.

1961-1961 wrote:

It says it is compatible for iOS 14 so not sure where I can find out more information

From Apple. This doesn't have specific updates, but I haven't heard of any iOS versions that weren't supported for every device that they started off with compatibility.

Supported iPhone models - Apple Support

You can check on iOS 12/13/14/15 for the list of supported devices, and the latest for the iPhone 6 is iOS 12.

The iPhone 6S is compatible with iOS 13, 14 and 15, the iPhone 6 is not.

1961-1961 wrote:

It says on google that it is

Then Google is mistaken.

You are welcome.