How do I update my iOS 12.5 to a iOS 13.0 on my ip 6?

Why not?

The simple answer is that the phone is too old. The more technical answer would be that the hardware on the iPhone 6 is not capable of handling the software demands of newer operating systems.

It is not possible, iPhone 6 is an outdated iPhone (9 september 2014), can’t be update to a higher iOS.

iOS 12.5.5 is last security updates for iPhone 6 -> released 23 september 2021.

Most apps require iOS 14 or later.

Considering a newer type of iPhone?

You can't.

Can I update my iPhone 6 past iOS 12.5?

as given to my granddaughter and needs to play some games with an updated version

Why not