how to do ios 13 i have ios12.6

Is not possible, sorry.

iOS 12.5.6 is the latest iOS security update for iPhone 5S, 6 & 6 Plus, the last security updates iOS 12.5.6 -> released August 31, 2022.

Only iPhone 6S / 6S Plus / 7 / 7 Plus / 8 / 8 Plus / SE 1st gen, 2nd gen & 3rd gen)  -> can update to iOS 15.6.1.

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.