Want to update my iPhone 6

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

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

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

The iPhone 6 is obsolete.

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.