How to delete pre existing code from arduino?

Hi,

I am curios, how to delete pre exsiting code so it doesn´t interfare new circuit before uploading new code?

You don't need to, when you upload new code the old code is gone.

so i need to upload the new code before I build the circuit?
maybe this question is to simple, but I am only 13 year old and I am new to arduino...

just upload blank code

void setup()
{
}
void loop()
{
}

I can't really answer without knowing what you are trying to do. If your old code will cause a problem for your new circuit then by all means upload your new code before making your circuit.

Edit or do as @squirrel_it suggested.

ok

thanks

if you want to update your code otherwise if you want your microcontroller without any code so upload blank one