how to change iPhone alphanumeric passcode to numeric passcode, even don't have option to change passcode.

You should be able to change the passcode/password to unlock your iPhone by going to Settings>Face ID & Passcode (Touch ID & Passcode), entering your passcode, swiping down, and tapping on "Change Passcode". If you do not have that option it either means that there are restrictions on your phone through Settings>Screen Time or that you have a profile on your phone, possibly through your employer, that prevents you from doing that. An employer profile might also prevent you from changing from an alphanumeric passcode to a numeric one.