how to reenter bwm 530i 1975 radio code

By preset, i am assuming you mean reset? There is no reset, once you have entered the wrong code the third time, it will lock you out The ignition is to be switched to R/first click (NOT IGNITION!!) for about 30-45 mins, best to attach a charger. This will then allow you to re-enter the correct code.

You need a code to make the radio to work again.