How to update iOS 13.4 on my iPhone 6?

Sorry. If that is an iPhone 6, it can't run iOS 13 and higher. If you want to upgrade to a newer version of iOS, you will need to buy a newer phone.

In case there's been a confusion in model numbers, the iPhone 6s can run iOS 13.4, but if you upgraded it now, you would not be offered that version. The phone would offer you an upgrade to iOS 15 (the last major version which it can run).

No way to update your iPhone model past iOS 12.5.7 version.

See why in this compatibility list:

iOS 12.5.7 is the latest iOS that an iPhone 6 can run

due to hardware restrictions. If you need a later version

of iOS, you will need to purchase a later model iPhone.

How to used iOS 13.4

I need the ios 13.4 in my iPhone

Uhm... impossible. If you want to update to a newer version buy a new iPhone, if you want refurbished! ( you can access to iOS 17 (18 this fall)