Want to update my iPhone 6

You cannot update an iPhone 6 past iOS 12.5.4. It it is not yet running that, go to Settings->General->Software Update to update it.

The iPhone 6 cannot be updated beyond iOS 12.x.x.x. It lacks the components to run newer software.

If it's already running iOS 12, you don't.

The iPhone 6 is obsolete.

The iPhone 6, released by Apple nearly seven years ago, cannot use an iOS version greater than 12.X.