Please how do I update my iPhone 6 without using WiFi???

You cannot install that app on your iPhone.

An iPhone 6/6plus can't be updated beyond 12.5.6, it doesn't have the hardware needed to run anything higher.

If you need a higher iOS version, you will have to purchase a newer phone.

You don't. You can use the web version, or buy a newer phone capable of running a newer iOS.