How to update my iPhone without computer

If your phone qualifies to be updated, it can be updated plugged in and connected to Wifi.

Your signature says you have iPhone 6 with iOS 12. If you are hoping to go higher to the current iOS 14.4, that's not possible, either via Wifi or using a computer. iPhone 6 cannot run anything higher than iOS 12.5.1 as it is now a 7 year old device which is no longer supported and incapable of running the newer iOS levels.

You can do it using Wi-Fi.

An iPhone 6 cannot update past iOS 12.5.1.